If by "case studies" you mean formal, written analyses this answer probably won't be of much help. If you're just looking for examples, the mobile / location-based social networking space is proving to be an interesting proving ground for game tactics. In particular, compare & contrast:
Newcomers to the space like Burbn are also adding elements of accruing points & leveling up, although it's unclear to what end.
This is a look at one space - I'm sure there are many other good examples, as well.