Sophie: You've given me quite a lot to think about there. I'd also like to congratulate you on managing to write a comment longer than the original blog post itself :)
Its true that I already keep track of most people in an online manner, either because they're online friends that I haven't met in person, or because they're friends that are now a long way away (you know who you are, hi guys). I've been using LinkedIn for a while and that is sometimes useful, but its oriented towards people's professional lives. Without email, blogs, MSN/jabber and IRC I honestly would lose contact with loads of people quite quickly, now you mention it. Thinking of Facebook as another interface rather than the miracle that the hype seems to make out, is probably the best thing.
I think I'm starting to get facebook though, I've got the aquarium thing and managed to lose about 20 minutes of my life to it yesterday! If anyone would like to send me a fish, I'd be unreasonably excited by that :)