By Jessica E. Vascellaro, Wall Street Journal

Facebook Inc. announced an ambitious plan to get its tentacles further out into the Internet by better linking people, places and things, as it looks to turn a massive audience into a pool of well-understood consumers.

A centerpiece of the changes involves a simple button, offered to other Web sites, that says “Like.” For free, other Web sites can install a Facebook “Like” button that users can click on to signal their interest in a piece of content, such as a band or an article. The user’s approval then shows up on his or her Facebook page, with a link back to the site.

The idea is that other Web sites will drive traffic back to Facebook.com, and in turn receive traffic from Facebook. Other sites can also offer personalized modules, telling individual users what their Facebook friends have done on the site, such as review a restaurant.

The new “Like” buttons transmit data about user activity back to Facebook. If they like a band, for example, a link to the band could appear in their interests. Since advertisers can already target ads to users’ interests, the new buttons could give advertisers more data to target ads to, but Facebook said it isn’t currently launching any new ad-targeting products in conjunction with the service.

Mr. Zuckerberg said Wednesday that the company has no current plans to sell ads on other sites.

Facebook said more than 70 partners have signed up to embed “Like” buttons or other customization widgets on their sites, including ticketing site Fandango.com and CNN.com.

In a keynote address to more than 1,000 attendees at a developers conference in San Francisco, Facebook Chief Executive Mark Zuckerberg depicted the technology involved as an expansion of Facebook’s vision and a new way to organize the information on the Web.

For years, he said, the Silicon Valley company has focused on tying together people on the Web, mapping out who knows whom. Now, Facebook wants to map out connections between people and their interests, gathering more explicit information about users’ favorites in order to deliver them a more personalized experience on Facebook and on other sites. He called it “the most transformative thing we have ever done for the Web.”

Six-year-old Facebook, based in Palo Alto, Calif., has become a fixture in many Internet users’ lives. Mr. Zuckerberg said Wednesday that the site has 400 million users and is growing at its fastest-ever rate.

As of February, it was the fourth most-visited Web site in the U.S., according to Nielsen Co. Each visitor spent an average of six hours and twenty-eight minutes during the month, also according to Nielsen, nearly triple the time spent on the next “stickiest” site, Yahoo Inc.

But Facebook’s advertising business, while growing rapidly, is still in its early stages. While the company is forecasting revenue of more than $1 billion for 2010, according to people familiar with the matter, it is only beginning to attract a meaningful part of advertisers’ budgets.

The announcements Wednesday are the latest sign of how the six-year-old start-up plans to sustain its momentum.

In an interview on the floor of the conference, Mr. Zuckerberg said “Like” is only one of many services Facebook may offer other Web sites in the future. “If we can offer something that we can do easily and a lot of people will use, it is worth it to us,” he said. He noted that other platform businesses, such as Amazon.com’s Web services business, got their start by taking features they used for their own business and making them available to developers once demand was great enough.

Facebook’s site has had “Like” buttons that allow users to endorse content like their friends’ status updates for some time.

Trip Adler, chief executive of publishing site Scribd, excited about the “Like” button, is flooding his company’s site with them, adding them on content pages and profile pages. While the site has long allowed users to share some content back on Facebook through a sharing tool, he said such content often flows through Facebook users’ news feeds before it gets noticed by many of their friends.

But after a user “Likes” content, it becomes a more permanent part of his or her profile, where it can be discovered by friends on a regular basis.

“It should in theory be a more permanent source of traffic,” he said.

Web sites that add the buttons can offer customizations for users, such as showing visitors what articles their friends liked or what restaurants they reviewed.

In a news conference after his keynote address, Mr. Zuckerberg and other executives stressed that the new services would not loosen its privacy policies. They said that Facebook won’t share any individual user data with Web sites that implement the “Like” button, but may share aggregate data like how many people “Liked” an item. Whether Facebook would share that data with a user’s Facebook friends would depend on the user’s privacy settings.

The social networking site said it was also getting stricter about what information in allowed third-party developers to access, moving to address a series of criticism from users and privacy regulators that its policies were too lax.

How many Web sites will rush to the Like and personalization technology remains to be seen. While developers were enthusiastic about the technology, others said they may wait and see how widely it is adopted before trying it.

Facebook said it would allow a group of pre-vetted Web sites, such as Docs—a new online service from Microsoft Corp.—as well as Yelp and Pandora, to see instantly personalized content from their friends, without requiring their users to log in or click connect.