In a counterpunch to the world's biggest online hangout, a small Web company called Power.com has sued Facebook, saying it doesn't follow its own policy of giving users control over their content.

Power lets users simultaneously access several social networks, including MySpace and Twitter. But Facebook isn't among them because the site has blocked Power. Last year it sued Power over the practice in a case that is still open.

In that lawsuit, Facebook accused Power of copyright and trademark violations and said the company gains unauthorized access to Facebook's computer network when it asks users for their Facebook login and password information.

This, Facebook says, violates its members' privacy and security, as well as its policy of prohibiting outsiders from asking Facebook users for their login information. Power, though, points out that its practice is a common one on the Web and even Facebook asks its own users to provide login information for their e-mail accounts if they want find their e-mail contacts on the site.

San Francisco-based Power filed a countersuit Friday in U.S. District Court in San Jose, Calif. Power claims that Palo Alto, Calif.-based Facebook improperly restricts its users' access to their private information when it prevents them from accessing it through a third party like Power.

Power's CEO, Steve Vachani, compared Facebook's policy to cell phone companies locking out third-party devices and applications from their wireless networks. To fight the case, the company has even hired Scott Bursor, a lawyer who successfully challenged this practice and obtained settlements.

"Data portability is an inalienable right," Vachani said. "Facebook has historically been the one company that has dragged their feet on this."

Facebook has its own platform for letting users access third-party sites using their logins for the social network. Facebook says more than 10,000 Web sites use the service, called Facebook Connect — though not Power.com.

While Facebook's approach in creating a "walled garden" is in different from other social networks that are more open to third-party access, it is unclear whether Power's claims can stand up in court.

Jonathan Zittrain, co-founder of Harvard's Berkman Center for Internet & Society, said some of Power's counterclaims won't fly.

"In my view Facebook isn't a monopoly," he said in an e-mail message. He added that much of the rest of Power's claims are of the "squishy" variety, and the court has leeway to decide what amounts to unfair competition.

In a statement Friday, Facebook said called the lawsuit "without merit" and said it plans to fight Power aggressively.

"We have made numerous attempts to work with Power.com but, after making commitments to comply with our policies, they continued to put Facebook user data at risk," Facebook said.

Zittrain called the debate over user data a healthy one to be having, adding that Power's lawsuit "will be among those forces pushing Facebook to maintain and possibly expand its openness."