I would consider Foursquare a successful "startup" but not yet a succesful "company". Twitter being the great grandpappy of startups I put in this group. They

I would consider Foursquare a successful "startup" but not yet a succesful "company". Twitter being the great grandpappy of startups I put in this group. They have a loyal and relatively large user base and a lot of mind and market share. They also have a feature set that most of their competition is now trying to emulate. They seem to be headed in the right direction but their success as a "company" is still to be seen. Also for perspective, I can't, off the top of my head, think of a mobile geolocation service that I would consider more successful then Foursquare. I could easily be missing something though.