I asked a coworker what it was. He said "I'm surprised you hadn't heard of it before. Over 60% of the world population is a member. Would you like to join?"
I said I'd be interested, and then he got a canny look on his face. "You don't have a LiveJournal, do you?"
I said yes. He sighed and said, "Sorry, pretend I never mentioned that. I don't know what the bumper sticker meant either."