I agree that one has to adjust for certainty and overconfidence, and peer disagreement does seem like a good reason to downshift as well.
OK, I see your point about shooting, agreed.
Regarding popularity, if that's the only criterion then also fAI would be a pet cause. I'd say it also depends on popularity among which group of people. Out of the smartest and most rational people I know, the majority of those who are interested in doing ethics, ie. in figuring out what "being altruistic" implies, would agree that animal suffering counts just as much as human suffering. And the smart and rational people who disagree are mostly not interested in doing ethics (in this way), as they just claim that it is all about "what they care about", a selfish defense that could just as well be used to uphold racism or sexism.
So on the above grounds I'd object to vegetarianism being a pet cause.
I notice that a large fraction of Effective Altruism people are vegetarian. This makes sense: in general Effective Altruists take moral issues seriously, even when that means changing your lifestyle. I'm not sure it's a good balance, though.
One way to think about this is to convert it into money. How much would I need to be paid to give up eating meat? All animal products? How much money would I need to spend on myself to be about as happy as I would be with less money but continuing to eat animals? I'd probably be willing to go vegetarian for about $500/year, vegan for maybe $2000/year.
It turns out you can probably pay to convince other people to be vegetarian much more cheaply than that. I estimate the cost of a vegetarian-year at $4.29 to $536 while Brian Tomasik estimates $11 with better methodology (which I looked at). This is by placing ads on facebook for a site where people can watch an animial cruelty video and ideally become vegetarian or vegan.
If you would get more than $11/year worth of enjoyment out of continuing to eat meat, why not give $11/year to convince someone else to not eat meat for you? Or give $50/year and be on the safe side?
(While you're giving money, you should probably give it to the organization that you think will do the most good with it, which I think is probably one of GiveWell's top charities. The nice thing about money as opposed to actions is that it's easy to redirect.)
I also posted this on my blog.