An acquaintance of mine, Kate, who's doing a grad degree in Internet Studies, has written a number of blog entries about Facebook and the way that it's changing our perceptions of internet socializing and expectations of privacy. She also makes various mentions of how it's altered expectations with regard to who is deserving of being a 'friend' on these networks.
Among other things, Kate writes that "suddenly, everyone’s grandmother, boss and ex boyfriend was using facebook and wanted to be your friend". Most interestingly for me, she - among many others, I should point out - also notes that it's now considered rude to refuse a friend request on Facebook from just about anyone. That guy that you only kinda knew in Grade 8 and haven't seen since? The girl from high school who you didn't even remember? A co-worker you don't particularly like? You can only decline them at the risk of looking like a jerk - which you will inevitably hear about, since you almost definitely have a half-dozen or more friends in common. It's like we're all captains of a schoolyard dodgeball game, picking teams from the kids along the fence and dreading that awkward moment when someone gets picked last.
This is in stark contrast to previous social networking apps - Friendster, Livejournal, etc. I can remember getting a Livejournal account in 2002 (I think) and my old roommate explained that he didn't want to lj-friend me - and it wasn't a weird, uncommon, or unexpected sort of thing to say. There was some expectation of privacy with these things - an expectation that's totally disappeared in the latest generation of social networks. Now I have my grandma and my parents on my Facebook.