Good debate! As a social media specialist I may be biassed, but I do believe in bands having a presence where their existing or potential fans are.
Before hitting the social media, a website goes without saying if you want to sell the band properly without having to post something new every 48 hours or so.
Everybody and their mother is on FB, so that's a good place to start your move into the social networks. Twitter, youtube, instagram, soundcloud etc. can follow if appropriate. It is important, however, to put a little effort in keeping your accounts constantly updated with a stream of posts of various kinds (I do that for a living), while at the same time making sure to avoid spamming or annoying your audience with too much irrelevant info.