As a 20-something living in SF, it's a daily thing now: I don't get invited to parties, I don't know about birthdays, I don't see my friends' photos, I don't have any contact with anyone from high school or college anymore.
There is a real social cost for someone in my situation to not be on FB. I struggle to quantify the harm, but it's there. I struggle too to explain to my friends why I'm not on FB. And yet I still think I'm better off without it.
The whole situation contributes to the isolation I already experience as an introvert and someone who doesn't much care for bars, clubs, or alcohol -- though I suppose I don't need to remind this audience that being alone != loneliness.
I guess it doesn't matter much anyway, since FB is still collecting information on me (and other 'shadow profiles' of users not on FB).