Foursquare has 4 million users, and has quadrupled in size since March 2010

"Testimony for that comes from Foursquare, which has now passed four million users, according to co-founder and CEO Dennis Crowley. In a conversation earlier today, Dennis said that it took a mere 50 days for them to grab a million additional users. The location-based service hit the 3 million mark in August 2010. It’s clear that Foursquare is starting to see a network effect — the more people sign-up, the faster it grows.
Foursquare hit the one million user mark in March 2010, a year after the service launched. The company has been making feature additions to the service that allow it to attract a more mainstream audience, and in an interview earlier this year, Dennis shared with me his grand ambitions of marrying the social web and location. Getting to 4 Million users is only part of that grand scheme."
Source:  GigaOM, 21st October 2010