To find the hottest restaurant, bar or concert venue in town, many young adults are no longer checking in with their friends. They’re “checking in” virtually via Foursquare, a location-based social networking site. Participants log onto the site and “check in” via smartphone to let contacts who are fellow users know where they are. At the same time, they learn what those users are doing — whether a co-worker is eating at the restaurant next door, or if friends are gathering at a nightclub across town. As “check in” alerts are traded between phones, the people attached to them instantly become aware of the spots that are popular in their social circles. Foursquare, which was founded by Dennis Crowley and Naveen Selvadurai, was introduced at the March 2009 South by Southwest music and interactive media festival in Austin, Texas. In recent weeks, the New York-based company has made headlines by gaining about 100,000 users in 10 days during this year’s South by Southwest event.…