ALEXANDRA HARCHAREK will never forget the first time she was anointed the “mayor” of the Benjamin Franklin Bridge in Philadelphia. It was an overcast Wednesday in April and she was driving to the city on her last day of an internship.

Never mind that she had to drive 15 minutes out of her way from her apartment in Sewell, N.J., to her desk at the Philadelphia City Paper. Or that it involved having to pull out her iPhone while weaving through morning rush-hour traffic to “check in” to the Foursquare application.

Once she tasted “mayorship,” she was hooked. “Being the mayor of a major bridge — how cool is that?” said Ms. Harcharek, 21, who currently runs a food blog, afoodcoma.com. She was so excited by her “nerdy achievement” that she posted a Twitter message about it.

Then, 17 minutes later, she was out of office.

While Foursquare has been talked about in corporate boardrooms as the next big thing in social media — it has some 2.5 million users — it has also spawned a more trivial pursuit: a petty and vicious battle over virtual pieces of turf.