Ten days ago Foursquare reached the two-million-users landmark, just three months after it had reached the one-million mark. A week earlier, the company received $20m in venture capital from a who's who of Silicon Valley luminaries. It appears the trajectory for Foursquare is only upwards. But as the critical mass of Foursquare users swells and intensifies over the coming months and years, the concerns over privacy are likely to magnify. In June, Webroot, a Denver-based internet security firm, surveyed 1,645 users of "geo-location-ready mobile devices", including 624 in the UK: 29% said they shared their location with people other than their friends; 31% said they accepted a friend request from a stranger; and, yet, 55% still said they were worried about their loss of privacy.