When Agata Slota's bicycle disappeared from a spot near Union Square one day in September, the odds were that she would never see it again. About 60,000 bikes are stolen in New York City each year, and only 2 percent of them are recovered by their owners. Because the bike was distinctive -- her brother had built it himself -- Ms. Slota, 27, posted an ad and photograph on Craigslist in the hopes that someone would notice it."I knew it was a unique frame," she said. "But I didn't really have my hopes up."The ad expired seven days later, and Ms. Slota and her boyfriend began planning to build a replacement.That is where it would have ended, except that a friend of Ms. Slota's had also posted a notice about the theft on an online chat room for fixed-gear bike enthusiasts. Such postings are common on the chat room, which serves mainly as a place for, in the words of one regular user, "geeking out over fancy bike parts." Weeks later, someone posted a response after spotting Ms. Slota's bicycle locked up outside a Quizno's in Midtown. This set off a flurry of messages that continued for weeks and, eventually, led to Ms. Slota getting her bike back.One of the people who read the post was Jack Drury, a 22-year-old former bicycle messenger who was working as an intern at Transportation Alternatives, an advocacy group for cyclists and pedestrians. Mr. Drury had never met Ms. Slota. But, according to another member of the chat room, he was "just itching to recover a stolen bike."Mr. Drury certainly does seem to put a lot of thought into bike theft. Ms. Slota's mistake, he said, was locking up to the horizontal bar of sidewalk construction scaffolding, which can be quickly unscrewed. It's better to lock up to the vertical post, he said. (Mr. Drury then went into a lengthy discussion of antitheft precautions, some of which involved different combinations of locks and others that involved ball bearings and glue, before lamenting the loss of a prized set of Japanese pedals that he had received as a gift.)The next day, Mr. Drury rode to the Quizno's, excited by the idea of helping a stranger recover her stolen bike. Ms. Slota's bike was not there, but on an impulse he walked into the store and began questioning the person behind the counter. The bike supposedly belonged to a deliveryman who had bought it for $200. Cajoling and threatening to call the police -- or to round up a posse of bike messengers -- Mr. Drury negotiated a deal to buy the bike back, then left the store wondering what he had done."You know when you're in a bike crash and you just go on instinct?" he said. "It was sort of like that." Jack Drury, the bike messenger who helped Ms. Slota recover her bike. Mr. Drury brought his news back online, where an increasingly eager group of friends started planning the next move. The suggestions went from the obvious (call the police or pay the man for the bike) to the vindictive (pay for the bike, but use counterfeit money) to the na