All through college and much of my early professional life, my rule for friending people on Facebook used to be that I'd only Facebook friend someone after I'd met them three times and that at least one of those occasions must've included the consumption of some kind of food or flavored beverage. It was a good rule. A solid rule. A rule designed to prevent me from Facebook friending frivolously while also ensuring that I didn't provide too high a bar to the Facebook friendship.
However, sometime in the last few years, I completely dropped this rule. Now I am an extremely frivolous Facebook friender. I friend people after meeting them once. Sometimes I friend people after speaking to them for just five minutes. On occasion, I friend people I've never met, if they're writers whose work I've enjoyed or whose online presence I appreciate.
I'm sure some people find this off-putting, but I just think, well...you never know, right? Maybe you never see that person again and in five years you have no idea who they are. But, on the other hand, maybe you don't see them for five years, but they comment on your wall and you comment on their wall and five years from now, you're both in the same city and you become best friends. You don't know. Anything could happen. The cost of being Facebook friends with someone is extremely small*, but the potential benefit is huge.
My Facebook promiscuity is all part of my (relatively new) policy of not trying to plan my future too much. It's way too much work to decide "Oh, I like this person and think they're Facebook-friend worthy" or "Oh, this other person is a fleeting contact who is not worthy of a Facebook friendship." Instead, I'd prefer to establish this extremely weak, tenuous linkage with lots of people and just see what happens! Sometimes the results can be wonderful and surprising.
I am a huge believer in the power of acquaintanceship. It's fun to have people who you only see once every six months or once a year or once every two years. People with whom you can get together and talk about that one video game you both like or that one person you both hate or that one awesome party. I think that too many people are way too focused on making very deep, lifelong relationships.
But you can't just have those relationships. Weak friendships turn into strong friendships and strong friendships turn into weak friendships. If you just dump all your weak ties, then you'll find, eventually, that attrition will leave you with fewer and fewer good friends.
Of course, there are also a number of sound reasons to not use Facebook this way. Some people put stuff on Facebook that involves their kids and families and travels and might have security concerns about giving that information to a bearded 6' 7" stranger with tired, sunken eyes (and an insatiable hunger for human flesh). Other people might just have less bandwidth to deal with other peoples' lives and might not want to see all my (hilarious) musings on _Orange Is The New Black._ That's totally fine. Some small but not-insignificant portion of my friend requests are ignored or rejected, and I don't take it at all personally. People have to use Facebook in the way that feels most natural to them.
*Some people seem to get super annoyed by the type and/or frequency of other peoples' Facebook posting. That never happens to me. I've never thought of myself as an easy-going person, but my Facebook wall almost never causes me any kind of aggravation.