By our calculations, Foursquare is now adding close to 20,000 users per day, up significantly from the estimated 15,500 per day rate near the time the startup hit 3 million members. This comes in spite of the startup's much publicized service hiccups earlier in the month.
About Foursquare previously reported that, judging by user IDs, Foursquare has already signed up its 4 millionth member. The user ID measurement is a close approximation, but the achievement will officially happen within the next few days, we hear.
Foursquare's impressive growth can be attributed to a number of things, most notably the release of version 2.0 for iPhone and Android, a Symbian release and celebrity attention from the likes of Conan O'Brien and hip-hop artist Big Boi.
Foursquare's biggest competition of late is coming from the social network behemoth Facebook, which has yet to reveal any stats around Facebook Places adoption. Gowalla, once neck-and-neck with Foursquare, was teetering around 450,000 users at the beginning of September.