As all the world knows, the Honorable Michael Nutter is the mayor of Philadelphia.

Not in the world of Foursquare, though. In that social-media world, the mayor of Philadelphia is some guy named "Frank S.," from New York.

In Foursquare, there's a mayor of the Ben Franklin Bridge, too - EricaLynn Gruenberg of Oaklyn.

The mayor of the Philadelphia Museum of Art is John Ingram of Lansdale. And the mayor of the NJ Transit train between Atlantic City and Philadelphia is Trisha Winter of Vincentown.

In Foursquare, people with smartphones register their whereabouts and share them with friends. You "check in" wherever you are - City Hall, Rittenhouse Square, the Liberty Bell, the Piazza at Schmidts. Almost any place that's a place, you can check in. And if you check in the most, you can become . . . the "mayor" of that place.

As in a video game, mayorship brings instant rewards. "Anyone else who checks in to that place," explains Mitch Rozetar, 21, an Art Institute of Philadelphia student from Sinking Spring, Pa., "is told you're the mayor."

Like Facebook, Twitter, and other social media, Foursquare helps friends stay in touch and groups come together based on shared interests. But it's also addictive, a mad game of points, badges, mayorships, and "specials" - discounts, free food and drink, free stuff - that encourage and reward the alert and curious. Says Rozetar: "Foursquare rewards you for going out of your comfort zone and trying new things."

The fame game

"There's a sense of fame to it," says Rozetar, mayor of Reading Terminal Market.

Brandon Thomas, 28, a restaurant manager for Jose Garces (and mayor of the Piazza), says, "I'll be walking through my restaurant, and a group will scream my name, 'Hey, Brandon!' . . . In shock, expecting the worst, I see Foursquare on their phones, and they say, 'It's great to meet the mayor.' "

"I use it pretty much daily," says Ingram, 26. Besides the Art Museum, he has the mayor's keys to 15 places including Steel City Coffeehouse in Phoenixville and the 40th Street beach in Ocean City, N.J. "I got my first mayorship and just took off."

Thomas' job naturally led him to the Piazza. "It took me two months to become the mayor," he says. "I've been living here for nine months, and it's a constant struggle to keep my mayorship. If I forget one day, I lose it."

"If you don't check in, others can 'oust' you as mayor," says Alia K. Dickerson, 24, a Temple University graduate student who was, last week, a mayor of Fairmount Park (some places can be listed more than once, with slight variations). "I 'check in' when I walk through the park, riding in the car with my mom, or on the 38 SEPTA bus."

Andrew Miguelez, 24, of Richboro, became mayor of Penn's Landing by checking in when he parked there for his job as a front-end Web developer for WebLinc. (He's mayor of 21 other places, including Pep Boys in Warminster.) "I was in a heated battle with another daily visitor. . . . The title of mayor bounded back and forth between us every couple of days as one of us would check in earlier than the other, or would check in on a weekend."

A sense of where we are

Foursquare, around since 2009, has 10 million registered users, according to its website. It's not making much money now, but in June it raised $50 million on a valuation of $600 million - so there are people who think it could. Advertisers might like it because people use it a lot: The 750 millionth check-in happened in June. It's used by urbanites, by men and women about equally. According to IgniteSocialMedia.com, its most active users are between 24 and 44.

For these users, it's more than just pointless competition. It's a way to stay in touch as you and your circle move from desire to desire.

"I usually use Foursquare to let friends know where I am," says Rozetar. Miguelez uses it to find nearby friends when he wants something to do: "I've found many more opportunities to meet up with friends than I would if I'd been unaware." Scott Webster, 31, of Washington Crossing, is mayor of Vetri and a 150,000-miles-a-year traveler: "It's neat to tell your friends where you are or where you have been." Other users track friends abroad, as Dickerson did with a friend in China.

Ingram says that through Foursquare he has "discovered places like the Italian Market and Girard House - off the beaten track, but well worth it." Drew Chrisner, 22, of Hamilton, N.J., stumbled upon the Mütter Museum. "It was awesome," he says. "I went a second time, to a masquerade ball there, and got the mayorship."

When you check in somewhere, Foursquare offers "tips" about what's available nearby. At the University of Pennsylvania Museum of Archaeology and Anthropology, tips praise the Chinese Rotunda and the Sphinx, and announce: "They throw great parties here." McFadden's at Citizens Bank Park is commended for its "beer tub girls." Webster uses Facebook for noshing tips, as does Gruenberg: "Foursquare has made quite a few dinner decisions for me!"

Free stuff

"A main motivation to become the mayor of a venue is the fact that you can gain special discounts," Rozetar says. Check in at McCormick & Schmick's, and you could get a free bar-food menu item with any beverage purchased during happy hour. And if you're the mayor, well, you get a free dinner entrée with purchase of any entrée of equal value or less.

At the Penn Museum's evening events, if you're one of the first 10 to check in after 5, a free drink awaits. P.Y.T. at the Piazza, one of the most frequently Foursquarely visited restaurants in the country, offers a free beer to checkers-in. (In January, its owner, Tommy Up, gave a free trip to New York to the 10,000th person to check in.) So does the Red Zone in Old City - but if you're the mayor, you can get your picture posted on the jukebox.

Location and community

Gruenberg, 33, says she has found Foursquare useful for business networking: "Businesses that have specials are usually more than happy to meet those of us who are using the app and cashing in on the specials." Ingram says that while he was mayor of Independence Hall, "people who visited there added me as a friend and asked for places to go."

One Philly community squarely behind Foursquare is the fooderati. Food workers often collect mayorships of bars and food venues. "We'll check in in the middle of rush hour," says Thomas. "We'll write something like, 'We're getting hammered tonight, 400 covers.' That's the craziness of the food world."

The place of desire

Ingram, an avowed "history and culture buff," celebrates his culture jones by checking in at Philly historic sites. We have met foodies Thomas and Webster. But maybe the best example is Gruenberg.

Gephyrophilia means "love of bridges," and it's safe to say Gruenberg is a gephyrophiliac. She is mayor of the Ben Franklin Bridge - but she has also checked in at 18 other bridges. She was hooked, she says, the day she became the mayor of the Longfellow Bridge in Boston.

"When I moved back to the Philadelphia area, it was a no-brainer for me that I would strive to become the mayor of the Ben Franklin Bridge," she says. "I knew it would be a difficult task, but I was up for the challenge." She checks in as she rides across into Philly on the PATCO High-Speed Line.

She admits the whole Foursquare thing can seem strange to neophytes. But Rozetar stresses his generation's "sense of connectivity": "We're used to sharing all kinds of information through Facebook, Twitter, Tumblr, etc." Miguelez calls Foursquare simply "mandatory."

Interviews with two local Foursquare mayors at www.philly.com/mayormarket and www.philly.com/mayorbridge

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