Facebook has been useful for us. It drives a lot of traffic to our site and presumably some of those people come to our event (we made an official facebook event so that we don't have a bunch of unofficial ones).
Oddly its a difficult way to communicate with members and build community. Facebook also requires payment to guarantee showing up in feeds now, for a non-profit its not worth it. A fair number of our younger and older artists just aren't on it (myself included). We're finding email is the best way to communicate whats going on with our members.
Facebook works best if Everyone is on it. Once fewer people aren't on it becomes less useful. I think thats its staying power currently. Its actually amazing that it runs at its scale. Myspace was popular, but it didn't have the nearly the volume of users as facebook (plus facebook is better at sharing photos, seemingly its reason for existing). If your not on it and your friends use it as an organizing tool, it can be a little isolating (Don't be that guy/gal who they have to go out of the way to invite to things...), but with more people not on it email seems to be returning as an organizing tool.