SAN FRANCISCO -- Facebook's plan to spread its online social network to other websites could be detoured by regulators looking into privacy concerns that have raised the ire of federal lawmakers.

Four senators said Tuesday that Facebook needs to make it easier for its 400 million users to protect their privacy as the site opens more avenues for them to share their interests and other personal information.

The Federal Trade Commission already had been examining the privacy and data collection practices of Facebook and other social networks, the agency confirmed Tuesday.

Then last week, Facebook announced a proposed expansion that irked Sen. Chuck Schumer, D-N.Y., and, he says, many Web surfers who called his office to complain.

Having built one of the Web's most popular hangouts, Facebook is trying to extend its reach through new tools called "social plug-ins." These enable Facebook's users to share their interests in such products as clothes, movies and music on other websites. For instance, you might hit a button on Levis.com indicating you like a certain style of jeans, and then recommend a movie on another site. That data might be passed along to other people in your Facebook network, depending on your privacy settings.

Facebook said all this will help personalize the Web for people. It stressed that no personal information is being given to the dozens of websites using the new plug-ins.

Still, it means that information that hadn't been previously communicated could get broadcast to your friends and family on Facebook.

And Facebook is indeed sharing some personal information with three websites that Facebook hopes will demonstrate how online services can be more helpful when they know more about their users. The sites with greater access to Facebook's data are business review service Yelp, music service Pandora and Microsoft Corp.'s Docs.com for word processing and spreadsheets.

Facebook users who don't want to be part of the company's expansion have to go through their privacy settings and change their preferences.

Schumer thinks the onus instead should be on Facebook to get users' explicit consent, a process known as "opting in."

Schumer sent a letter calling for simpler privacy controls to Facebook founder Mark Zuckerberg. The concerns were echoed by Sen. Michael Bennet, D-Colo; Sen. Mark Begich, D-Alaska; and Sen. Al Franken, D-Minn.

Facebook tried to assure Schumer that its latest idea won't invade users' privacy.

"We welcome a continued dialogue with you and others because we agree that scrutiny over the handling of personal data is needed as Internet users seek a more social and interactive experience," a Facebook vice president, Elliot Schrage, wrote in a letter to Schumer.

Schumer called Facebook's response inadequate and said his staff planned to meet with the company today.

Meanwhile, the FTC indicated it will weigh into the debate at some point.