I do this for one simple reason: Facebook is too valuable for me. If I have hundreds or thousands of friends, the service simply isn't that useful; I get bombarded with too many status updates, and after a while I simply stop caring. I haven't yet determined the sweet spot for the ideal number of friends on Facebook, but I'm pretty sure that - for me - it's under 100.
Of course, Facebook is just one example; as our Adam Ostrow had shown in a recent post, by tweaking Facebook custom friend lists you can make sure that you don't see stuff you don't want to see, and by tweaking the privacy options you can hide your personal details from certain groups of Facebook friends. However, Facebook is just one social media site among hundreds, and not all of them offer such detailed customizing options.
On Twitter, I'm a bit more relaxed, and I follow several hundred people. But even there I'd rather have 100 relevant friends than 10000 random ones. It's simply practically impossible to actively follow that many people.
Of course, if you're a social media consultant or otherwise deeply involved with social networking & media sites, you might find it a necessity to amass as many friends as you can on sites such as LinkedIn, Twitter, Digg or Facebook. From a certain perspective, I should probably be doing it, too. But I just can't get myself to do it; for lack of a better word, I like Facebook too much to ruin it by having too many friends. Sure, if you look at Facebook and Twitter as somewhat of a passing trend which might be gone in a year or two, none of this matters. But I believe these sites are here to stay, and I'd rather take it slow, even at the cost of not being the hippest user out there.