Foursquare is now experiencing 40% growth per month, says Eric Friedman, Director of Client Services at the popular location-based social network. It now has 1.6 million members, so, if our calculations are correct, the company plans to add 640,000 members this month. Guess all those big partnerships with Bravo Media, Zagat, Warner Bros., and HBO are paying off. Back in April, top VC's were valuing Foursquare at $100 million, according to Business Insider -- but not because they thought it would be the next Twitter, or that the location-based social network can crack the local advertising nut. On the contrary, Foursquare is so prized because there are so few viable startups at the moment, sources told the Web site.