I, personally, would start with Facebook and branch out from there. Agreeing with
@Domingotarlejr, I get the impression that most people have a Facebook account and use it at least fairly regularly, while other social media like Twitter and Instagram have less of an "everyone uses it" feel (though this may be because many of the people I interact with are ages 25+; I couldn't tell you off the top of my head where you'd want to start for a younger audience). I also find that Facebook makes it easy to promote or engage the audience: you can create individual event pages for events, people can leave reviews, it creates albums for pictures, makes it easy for people to follow because most already have a Facebook account, etc. I think the flexibility is really important, and the extra features put it a step above other social media. Google+ is similar, but the lower usage numbers make it significantly less practical and useful. You have to go where the people are, and right now, I think (if you had to choose one), that'd be Facebook.