Ten months ago, New York City launched the first public bike-sharing initiative in the U.S., backed by $41 million in Citigroup money meant to help refashion the company's image after the financial crisis. The program has been struck by nagging logistical and financial problems, and simultaneously been a wild success. In fact, its popularity led to some of those early teething issues: With 100,000 annual members, it has netted far more riders than expected.

"This program has capitivated New Yorkers like something I've never seen," Elyssa Gray, the director of creative media for North American marketing at Citibank, said at the Ad Age Digital Conference on Tuesday.

New Yorkers have not just been hoping on the bikes, to Citibank's delight. They've posted photos and videos of the rides on a range of social media outlets, each splashed with the Citibank logo. Ms. Gray said the bank has netted $4.4 million in earned media since the launch.

"Going into it, we knew there is always a brand risk," she said. A particular frustration is debunking the idea that Citibank is directly managing the program. "But with risk comes great reward, too."

Each of the marketing metrics Citibank tracked in the program's initial three months posted double-digit growth, Ms. Gray said. Brand preference ticked up by 25%. When asked if Citi Bike would pop up elsewhere, Ms. Gray did not rule it out. "A lot of cities are looking," she said. "Trust me, I get a lot of phone calls."