The secret sauce may lay in the business model. Selling apps at 99c or more doesn't seem to be profitable. Giving them away in a freemium model however, may prove worthwhile. Social games do really well with virtual goods. I don't know how Foursquare makes money, but they seem to be doing alright. With regards to billion dollar app companies, ngmoco was acquired for $400m, not quite a billion but pretty damn good for a company that publishes a bunch of little mobile games.