I Can Do It is a site that integrates with facebook and encourages people to commit to activities, related to the 5 ways to wellbeing. See www.icandooit.com for a short video tour. 43% of UK population have a Facebook account - only Hong Kong and Canada have a greater proportion of citizens as users!
Users choose activities from a default list for each of the 5 Ways to Wellbeing (keep active, connect etc) or they type in a bespoke activity. Commitments are auto-reposted as a facebook status update so friends can provide messages of encouragement (and hopefully start to use the app themselves).
Users can signal also when they've completed an activity and this 'success' is again re-posted to fb so friends can congratulate them.
Facebook's advertising system allows very targeted adverts by location, age, gender, declared activities etc and the facebook analytics offer detailed use reports.
It's currently a v1.0 release done on a shoestring budget, but we're in discussions with several MH Trusts and PH people in the North West and hope to find a bit of money to smarten it up and add a few more features, especially in terms of tracking, anonymously, what kinds of activities people are committing to so further useful info can be provided to users.