Facebook's mobile app install units are a hit. In spring 2013, Facebook had announced that 25 million app downloads had been driven through the iTunes App Store and Google Play. At the time, over 40 of the top 100 top grossing apps for both iOS and Android were leveraging Facebook's mobile install units.
The unit's success makes sense because:
1) those applications are natively integrated with Facebook (so it's efficient, familiar and easy) 2) they are social and data rich (ie "150 friends are using the application") 3) ... which in turn means that the game / application itself is inherently social and welcoming
Yesterday, I saw the following unit for the first time, which is a natural extension of the install unit... but driving traffic around media. The unit promotes the movie Elysium and links directly to iTunes. As Facebook experiments more and more with ratings, reviews and public content (ie hashtags, celebrities, etc) - this becomes more and more engaging.