Facebook is quite powerful at lubricating relationships between acquaintances. Too powerful, sometimes: the social network turned an English garden party into a riot requiring a small police army to put down.
About 200 youngsters showed up, not all of them intent on behaving themselves.
Up to 70 police officers from two forces, including dog handlers and a helicopter team, were needed to break up the party,
Keep in mind that the teenagers who organized the event didn't even include their address in the invitation, and had the apparent cooperation of parents. But Facebook + teenagers + alcohol = "Lads were jumping over the fence from other gardens and we ended up with about 150 in our garden," as one of the parents put it.
Far better is something with some built-in social friction, like a written invitation, or a painful-to-use website like, say, eVite!