Run small contests, sweepstakes or games.

Posing a question or running a small contest draws followers to your site. Giveaways are one idea; incentivizing social sharing is another.Possible strategies here might involve a "like to win" contest, or the launch of "a selfie" competition. Alternatively, you could design a voting contest that would encourage your followers to engage with one another and with you. Spell out the rules and parameters of your contests, and let your followers take it from there.