It appears that Foursquare hit the three million user mark sometime over the weekend. The location based social network has been adding users at an incredible rate and was only at 2 million back in early July. Let’s do the math… that’s 1 million users in a month and a half! It took a year to reach one million, three months to his two million, and now a month and half! Crazy! So at this rate, they’ll reach 4 million in 3 weeks? Watch out Facebook! Some thought Foursquare was doomed when Facebook rolled out its location-based feature, Places, a few weeks ago. But Foursquare’s co-founder Dennis Crowley stated that Foursquare had its biggest day of sign-ups following Facebook’s announcement; and Crowley told the LA Times recently that the start up is growing at about 180,000 users every 10 days. No, seriously; watch out Facebook!