Quentin, I worried too about the "few friends = low status" thing when I started on Facebook. But speaking now as an old hand I'm fairly confident that the only people who make such judgments or worry about them are newbies!
And yes, you CAN hide who your friends are.on Facebook. There are many other privacy settings as well. It would be too complicated to go into it here but they have a Help Center which will tell you how. You can find the Help link on the menu that will open up when you click on "Account" (at the top right-hand of any page) or, in small letters, at the very bottom of any page on the far right.
It's OK to ask someone you just met to friend you.
Not only do some people friend every last acquaintance, it's also common to friend people for the purpose of game play (there are numerous game applications you can access through Facebook, and for one reason or another it's often advantageous to play with people who are friends, so people will friend one another for the sake of the game). Then there are people who friend friends of friends because of shared interests or whatever. Bottom line: If somebody has 1,000 friends, nobody assumes that he is best buds with all those folks in real life.
Don't worry too much about the etiquette--if you spend some time with it you'll pick it up. Most people will be happy to help you out if they can (though a lot of people don't know about all the privacy settings. They're really not hard to set but you have to look for the info.)
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)