Social discovery using a game similar to Sims plus curation principles from Pinterest to encourage everyone to add content. 75% of users are female whereas 70% of Foursquare's users are male. Great long-term user engagement and growth metrics.
Posse lets merchants interact with users who've recommended them by enabling them to send gifts, messages and offers through the system. Posse has already signed up 14,000 merchants while in private beta (4000 in US).