I think pretty much everyone who uses Facebook is, at least partially, in this mindset. The network is so large that people feel as though they're missing out if they leave (whether or not that is actually true).
I signed up again, after a 2+ year absence, because I discovered solid proof that I was missing out. People would be surprised not to see me at some event or other, and I would comment that I hadn't been invited, and this would create confusion - "well, let me check facebook, I'm sure I invited you" - "I'm not on Facebook" - "Ohh....."
One of my friends had a special list of half a dozen people he still sent emailed event invitations to. It's extra work, though, and most people stopped bothering. I decided that my hate for Facebook inc. was not strong enough to make me willing to inconvenience my friends, so I signed back up.
When people wanted to organise birthday parties at my last office they just added it as an appointment in the calendar. I suspect that if most email clients ever are well integrated with calendars then that'll probably be the end for facebook. ATM, however, there's no good calendar software integrated into most email clients so arranging a meet - even if you've got the social connections stored on your phone - is a pain in the neck.
