For a comic like yours, I would say to also post on Tumblr and Smackjeeves. A public Instagram account for your art or a deviantart account can also be great ways to gain followers. Crosspost on a bunch of different sites. Facebook is generally good for short comic strips rather than a long comic with a plot.
And as the other commenter said, paid advertisement never hurts! But if you don't have the money for that, building a solid network of followers and friends on social media can be just as good!