Most brands only have a basic understanding of who their Facebook fans are. They can see gender, age and location breakdowns through Facebook’s Insights tool, but they’re hard-pressed to find out much else, particularly their other interests. Coca-Cola knows its fans like the soda brand, but what about the TV show it sponsors, American Idol?

A number of social marketing firms like Wildfire, SocialCode and Relevvant have created workarounds—typically asking users to give a brand permission to access their interest graph when they sign up for the brand’s Facebook app—but marketers are limited to that subset of their fans. Well, most marketers are limited.

It turns out that Facebook has been internally allowing a select number of marketers to see their fans’ other affinities, such as their favorite brands, bands or TV shows, according to sources with knowledge of the tool. This new brand affinity tool would theoretically enable a company like Macy’s see the most popular TV shows among its Facebook fans—which could inform media planning.

Another use case: a brand like Ford could see its fans’ favorite bands, which might help with sponsorship decisions or even what songs might end up in future Ford TV ads.

Adam Lehman, COO and general manager at data management platform Lotame, said the tool would be a great example of harnessing quality social data because of its first-party nature. And in this case, brands would be able to act on that audience data for more than ad targeting on a broader strategic level.

But Lehman doesn’t need to tell Facebook about the value lying in its data. The company's fully aware. In 2010 Pete Warden, founder and CTO of travel app Jetpac, developed a similar tool that scraped Facebook, looking for interesting data that people could play with, hoping to see connective patterns among users. Facebook threatened to sue him.

Given that the new tool would let brands access such highly prized insights—and given how aggressively Facebook guards that information—it’s perhaps not surprising that Facebook has been keeping it under lock and key. The company has only been using it on a one-off basis with big-budget brands—or “priority accounts,” as two sources phrased it—and agencies and their clients had to visit a Facebook office in order to access it, sources said.

A Facebook spokesperson said the company had nothing to share at this time. UPDATE: Facebook has provided the following statement: "To help marketers build better campaigns, Facebook offers aggregated insights to managed clients that help them understand trends about their fan bases. These tools do not provide marketers with any data about their competitors' fan bases. As always, Facebook does not share user-specific data with advertisers."

Should Facebook ever decide to roll out the brand affinity tool publicly, brands will come running. Jeff Widman, who runs the Facebook analytics company PageLever, said the brand affinity tool would “definitely” be a valuable feature for brands.

“If you could see what other pages your fans have liked, you could make your posts more engaging by writing about those other pages. It's also an easy way to identify potential sponsorship or Facebook co-promotion opportunities," Widman said. "If you're trying to grow your fan base, you can run Facebook ads at people who are not yet fans of your page, but already fans of related pages. We’ve had multiple customers request such a feature.”