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.