Still thinking about building a social-networking site for your employees? Sure they haven't started without you? Check Facebook, you might get a surprise.

As companies grapple with whether, and how, to offer a social-networking platform for their workers, some are realizing that if they don't act quickly, their workers will go ahead and do it anyway. And that can mean forfeiting control over what content gets posted where, and who can see it.

"Do you really want Facebook to manage it for you in the outside world, or do you want to do it yourself so you have control?" said Duane Nason, a lead Web engineer with The Gap clothing retailer. "If someone posts something to MySpace and you want it taken down, what's their policy on that?"

Nason was at the Web 2.0 Expo this week to learn more about how social networking, mashups and other new technologies can be applied at his company. He's one of many representatives from large companies at the conference who are grappling with similar questions.

Nason, who helps run The Gap's Web site and intranet portal, threw up a wiki recently for use by his small circle of colleagues. He was surprised to discover later that other employees in the company -- mostly younger staff in junior positions, he said -- had been entering their profiles and using the wiki, even though it hadn't been marketed internally. "They just found it and started using it," he said.

He also noticed that, as with many large companies, employees at The Gap have started to form their own groups on Facebook. Big companies should develop a social-networking strategy before their employees do it for them, he advised.

An IT manager with another well-known retail company agreed. "If you don't do it for them, there are enough tools out there that they're going to do it themselves," said the manager, who asked not to be identified because he is not authorized to speak publicly for his company, and because his company is still unsure of its strategy.

He was at Web 2.0 to explore ways of using social networking to build a closer bond with customers. E-commerce sites are not good for building a community, he said, but a social network could allow customers to form their own groups and discuss products they are interested in. He is also interested in widgets that provide updated information about special offers, for example, to a user's desktop.

Among the issues for the retailer is whether to build its social-networking platform on an established site like Facebook, or whether to set up an independent site with the retailer's own brand. External sites like Facebook and MySpace might become less important if big companies can attract audiences to their own sites, he said.

The retailer has only just started to explore these issues. "It's still very early; we're still in perspective-gathering mode," he said.

An assistant marketing manager from a U.S. tobacco company also was at the show to explore how social networking could help attract a community around his company's products. Some smokers are passionate about their brand and have registered at the company's site, helped by offers such as vouchers for cheap cigarettes, he said.

However, tobacco companies face particular challenges. The U.S. government has been passing tougher regulations for the industry, and some tobacco companies expect new laws that will require them to ensure that visitors to their Web sites are of legal age, by collecting names and even Social Security numbers, said the manager, who also asked not to be named.

"We have to be very careful about who we attract to our site," he said, adding that new regulations could make using third-party Web 2.0 platforms more difficult.