As far as I can tell, the only 100% accurate tracking is occurring with Facebook Mobile App Install ads since they have an integrated solution with the App store. But there are many 3rd parties that claim to do it, but my understanding is that it's a "correlative" confirmation, not a true confirmation. Which is the best?
The problem is that nowadays everybody wants you to install their api. While this is easy in the beginning, in the end it becomes a pain in the #$%#.
You can install http://www.flurry.com/ analytics on your app and be able to know the conversion to installs even for every creative you have on every social network or even links you give offline.
Really helped me track every possible detail on app promotion and the best of all... It's free ;)