In my experience, Americans are actually eager to talk to strangers and make friends with them if and only if they have some good reason to be where they are and talk to those people besides making friends with people.
A corollary of this is that if anyone at an [X] gathering is asked “So, what got you into [X]?” and answers “I heard there’s a great community around [X]”, then that person needs to be given the cold shoulder and made to feel unwelcome, because otherwise the bubble of deniability is pierced and the lemon spiral will set in, ruining it for everyone else.
However, this is pretty harsh, and I’m not confident enough in this chain of reasoning to actually “gatekeep” people like this in practice. Does this ring true to you?
It looks to me as if, of the four "root causes of social relationships becoming more of a lemon market" listed in the OP, only one is actually anything to do with lemon-market-ness as such.
The dynamic in a lemon market is that you have some initial fraction of lemons but it hardly matters what that is because the fraction of lemons quickly increases until there's nothing else, because buyers can't tell what they're getting. It's that last feature that makes the lemon market, not the initial fraction of lemons. And I think three of the four proposed "root causes" are about the initial fraction of lemons, not the difficulty of telling lemons from peaches.
Note that I'm not saying that "drugs", "MLM", and "screens" aren't causes of increased social isolation, only that if they are the way they're doing it isn't quite by making social interactions more of a lemon market. (I think "screens" plausibly is a cause of increased social isolation. I'm not sure I buy that "drugs" and "MLM" are large enough effects to make much difference, but I could be convinced.)
I like the "possible solutions" part of the article better than the section that tries to fit everything into the "lemon market" category, because it engages in more detail with the actual processes involved by actual considering possible scenarios in which acquaintances or friendships begin. When I think about such scenarios in the less-isolated past and compare with the more-isolated present, it doesn't feel to me like "drugs" and "MLM" are much of the difference, which is why I don't find those very plausible explanations.
You're mistaken about lemon markets: the initial fraction of lemons does matter. The number of lemon cars is fixed, and it imposes a sort of tax on transactions, but if that tax is low enough, it's still worth selling good cars. There's a threshold effect, a point at which most of the good items are suddenly driven out.
Funny thing is that your chances improve when you start actively approaching people. A random person you call is much less likely to be involved in an MLM scheme than a random person who calls you.
Hence the advice to lost children to not accept random strangers soliciting them spontaneously, but if no authority figure is available, to pick a random stranger and solicit them for help.
Assuming you're the first to explicitly point out that lemon market type of feature of 'random social interaction', kudos, I think it's a great way to express certain extremely common dynamics.
Anecdote from my country, where people ride trains all the time, fitting your description, although it takes a weird kind of extra 'excuse' in this case all the time: It would often feel weird to randomly talk to your seat neighbor, but ANY slightest excuse (sudden bump in the ride; info speaker malfunction; grumpy ticket collector; one weird word from a random person in the wagon, ... any smallest thing) will an extremely frequently make the silent start conversation, and then easily for hours if the ride lasts that long. And I think some sort of social lemon market dynamics may help explain it indeed.
— Groucho Marx
Alice and Carol are walking on the sidewalk in a large city, and end up together for a while.
Carol thinks:
Carol might not think all of that consciously, but that's how human social reasoning tends to work.
In most cases, Alice understands this and doesn't even try to talk with Carol in the first place.
lemon markets
Economics has the concept of a "lemon market". When sellers have more information about item quality than buyers, and reputation isn't a factor, sellers will try to sell low-quality items, and consumers will respond by driving down prices until only low-quality items are worth selling.
I think a similar thing has happened for social relationships in many countries over the past several decades.
An increase in social isolation over time has been widely noted; Bowling Alone was written in 2000, and the issues it described have increased since then. My view is that the above dynamic is responsible for much of that change.
What are the root causes of social relationships becoming more of a lemon market? There are many causes, but 4 causes I consider significant are listed below.
urbanization
In many countries, people have rapidly moved to bigger cities over the past several decades, and this affects social dynamics.
Human relationships generally have some upfront costs and then a net benefit over time. In very small towns, your chance of having a relationship with a random resident is higher, so the initial upfront costs are amortized over a higher probability of establishing a good relationship.
In a very small town, if Dave dates 3 girls at the same time and treats them all poorly, afterwards, every girl in the town will know to avoid him. Bad behavior is less attractive when random people you encounter in the future are more likely to be aware of it, and bad behavior being less likely means relationships with random people you meet have a higher expected value.
drugs
If you're "friends" with a heroin or fentanyl addict, and you have some money and nice stuff, they will steal your stuff to buy drugs.
The US has seen a substantial increase in the number of drug addicts and overdose deaths, first because of prescription opioids and now because of fentanyl.
MLM schemes
Sometimes, when someone in America seems like they're trying to make friends with you, it's actually because they want you to join some MLM scheme they're involved with, so they can make money off you. Participation in such schemes has grown over time, and they've become more aggressive.
screens
Suppose 2 strangers meet at a park. They could talk to each other, but that option competes with whatever they'd otherwise do. If they're both addicted to mobile games, or both listening to music, the odds of them having a conversation are low.
Visiting friends' houses and talking to them used to be one of the main things people did for fun, but television gave people entertainment that didn't require any other people.
possible solutions
smaller communities
The effects of urbanization can be partly cancelled by putting people in a small community. Ideally it would select for compatible people, but even a random selection seems to work. The biggest examples of this are students making friends in high school and employees making friends with coworkers.
excuses for socialization
In my experience, Americans are actually eager to talk to strangers and make friends with them if and only if they have some good reason to be where they are and talk to those people besides making friends with people. If there's a situation which can attract people who don't need to be there to make friends, that's no longer a lemon market.
The biggest example of this is probably employees making friends with coworkers, but there are many other examples, such as:
Video games have become more and more popular over time. The main problem with that last item is that most people who meet via video games are far away. Even if games tried to match people in the same city, very few games have enough players to make that an option. I think local LAN tournaments or cooperative challenges could be a community-building thing, to some extent, but the fragmentation of the games and TV shows and other sorts of entertainment people like has increased.
software
Computers are fast. Algorithms have been developed extensively. Companies like Google and Facebook have lots of information about people. Maybe software can just match compatible people?
But...what we actually have is people swiping on Tinder. Guys lying about their height and income and swiping 1000 girls. Girls using photo filters and all going for the same few percent of guys.
OKCupid tried to algorithmically match people based on various qualities, and some people liked it a lot, but then it got bought by Match Group in 2011 (for $50 million, which isn't very much money here) which changed it to be more like Tinder.
Maybe Youtube could match single people based on the videos they liked and watched? I don't see that happening, but it seems theoretically possible.