Location service Loopt just signed up its four millionth user, with daily sessions up 60% since the launch of the iOS 4 software for iPhone, which allows apps like Loopt to run in the background. Meanwhile, the much-hyped Foursquare has just signed up its two millionth user, just after closing a $20 million funding round. Gowalla originally one of the key social location startups – has been involved in a big billboard campaign overlooking Madison Square Garden in New York, inviting people to check-in. And how many have? 64.