Wikipedia
Fan-gating
Fan-gating (also known as "Like-gating") is the practice of acquiring more fans for a Facebook page by requiring Facebook users to "like" the page in order to access specific content associated with the page. This content is typically exclusive features, promotional offers, games or other material.
On August 8, 2014, Facebook updated their platform policies to forbid businesses and developers from using like-gating on new apps, while existing apps will need to comply by November 5, 2014.