What about some little community challenges? Something like for every 50 additional "likes" on facebook, we get a new screen of an upcoming map or something, or some little more Info. But beginning with the first screen at 500 or 1000 likes or whatever, so make it at first a challenge, with a reachable goal.
I saw somehting like this on another mod, they got more then 1500 likes since then.
Just a little example of what is possible.
PS: I dont have a facebook account, but for this I would think about it.