I don't really use Facebook much, so it's rare that I ever see an incoming friend request on there, but in any case, I wouldn't add anyone unless I at least know who they are, since Facebook is a much more personal space than any of the forums I go to are, and adding people on there pretty much lets people into your personal life. Not something I'd let any random person who adds me into ;p
As for PC and pretty much any other forum I've ever been on, I'll gladly accept any friend request, as long as I don't have any problems with the person of course. I will occasionally go through my friend list and delete anyone if I haven't seen them in a while and didn't know them personally/didn't see any chance of them returning. I'm not one who really adds other people that much, and I think that's mainly from a subconscious fear of rejection, or the thought of someone not actually wanting me on their friend list. It's not something I've ever really experienced before, so it's rather unjustified, but yeah, can't help it :( But yeah, I'll probably take the initiative if I've been talking to someone for a while and if they haven't done it before me :p