I'm not sure I can add anything to you but would love to follow your progress.
I spent the last year in the space (Social/Local/Message/Activity/Dating) and have learned a lot about maybe what not to do :)
My only recommendation is just build the product and get it into users hands because no matter what you build or thing people want its going to change, maybe a little, maybe a lot.
Marketing might be simple, do what Facebook did but not what they do now. Get out to small groups but market to large markets (lots of small groups).
Its all about cool features, a simple app is so 2013, people want cool features and cool UI.