Finish the game and build followers. Pay Facebook for post promotion.

Note that you don't get good value for money from Facebook, you'll get much better value for your money from Google ads and twitter advertising that can be targeted much more accurately.

Good luck!

Oh and if you haven't even got anything to show yet don't bother, but if you have at least a demo, YouTube videos or something more than just concept art, start building followers now.

Set up a development journal here for free and keep it updated, as a bare minimum...