You log into Facebook and you have a birthday reminder for one of your "friends." The name doesn't ring a bell and the profile picture isn't helpful. Someone you knew in high school who got married, maybe? You have no idea.
You click on their profile. Their photos look kind of vaguely familiar, but you still can't place them. Eventually you end up digging into their friend list like some kind of stalker to see your friends in common, and you realize it's your friend's cousin's ex-fiance you shared a table with at that wedding four years ago.
This is friendship?
How many of your Facebook friends could you pick out of a crowd (and know their names)?
A more subjective follow-up question for the comments: How many of your Facebook "friends" do you feel like are actual friends? The kind you'd stop on the street and catch up with?