It's interesting. Despite the fact that I have 66 friends, I only ever regularly keep in touch with one of them. Whom I've probably pissed off by this point due to my irregularity with messaging and long periods of time without messages of any kind. You know who you are.
Anyway, a lot of the people who are my friends on this site I befriended when I was in my Forum Games & RP phase, so I did keep up with chatting between them then. Ever since, though, I find myself talking to people on my friends list less regularly, though I usually befriend people myself when I have a long enough discussion with them and they seem interesting enough.
As for the the odd person or two who just randomly befriend me, I don't really pay much attention and will just add them regardless. They probably have some reason for befriending me; they read my posts or something and agreed with what I had to say at some point. I think it's less being a friend of someone in that case and more a fan of someone (like it is on Kongregate). I'm guessing that's kind of similar to what happened in the cases of [user]sv93[/user], [user]Neonbob[/user] and [user]MaxTheReaper[/user], all of whom resulted with over 200 friends due to what could be seen as more of a fanbase than anything else (though for the record, bob added me as a friend before I added him as one and Max I befriended after a short conversation about his 10k post).