The Pages are free to set up, and allow Facebook members to sign up as "fans" of a brand or product or as supporters of a political candidate. More than 150,000 Facebook Pages have been created to date.

Among the top 25, Obama's is by far the biggest draw with 834,550 supporters--nearly double the 426,635 fans for the next most popular site for "The Chris Moyles Show," a popular BBC Radio 1 program. (The U.K. is the biggest Facebook market outside the U.S., with about 9 million users.)

Rounding out the top three is an Apple Inc. page geared toward college students--with 415,535 fans, according to the list compiled by the blog Inside Facebook. The site features the company's discount program for students and faculty, including access to Apple Stores customized for participating schools.

Taking fourth place with 350,983 fans is Victoria's Secret Pink, a product line from the lingerie chain aimed at high school and college students.

The only other actual brands with among the top 25 Facebook Pages were the NBA (17) and Nutella (21).

Music marketing dominates the list, with 40% of the pages dedicated to bands and singers including Coldplay, Avril Lavigne, Justin Timberlake, Chris Brown and Dave Matthews Band, to name a few.

That's not surprising, given the pervasive role music has played in social media. Seven of the top 10 most-viewed videos on YouTube to date are music-related, and MySpace first gained popularity among indie rock bands trying to promote themselves online.

While Facebook says more than half its users are now found outside college campuses, its top brand pages are mainly pitched toward the youth demographic. That includes the site for Obama, whose campaign has drawn especially strong support from people under age 30.

What about his presidential rivals? Hillary Clinton came in at no. 11 with 156,352 supporters, while John McCain was 23rd, in a near-dead heat with British soap opera "Hollyoaks" and Nutella with 127,143.

Because Facebook Pages don't represent paid advertising in themselves, the direct benefit to Facebook from the program is difficult to gauge. Marketers can place Social Ads in updates sent to friends about fans' interactions with their sites.

But among the top 25, only a handful of major brands, like Apple and Victoria's Secret, would be likely to consider that option. Like the thousands of applications on Facebook, the brand pages provide free promotional vehicles for advertisers.

Facebook's bet is that by expanding its audience and user engagement through such initiatives, it will ultimately find ways to attract more ad dollars to the social network.

"The traffic on social networks seems to be holding steady, so eventually the advertisers will have to come around," wrote Gartner analyst Andrew Frank, in a post on the research firm's media industry blog Thursday. "That makes the social sites of 2008 fundamentally different from Pets.com in 2001."

Earlier this week, eMarketer cut its revenue projection for Facebook in 2008 to $265 million from $305 as part of a broader downward revision of its forecast for social networking ad spending to $1.4 billion, from $1.6 billion.