To encourage the behaviours you want from your visitors, gamification can be a useful growth hack. In addition, it keeps them engaged and eager to return for more.
When they released their first book, Foundr did exactly this. They held a contest for people who signed up for their email list and their social media accounts before the release.
Foundr upped the ante by rewarding them with points for doing things like tweeting about the contest or recruiting friends and family to join the competition, rather than simply asking them to fill out a form to join their contest. Foundr received thousands of new email subscribers and achieved four times their targets as a result.