Facebook warns advertisers: You might not like 'clear history'

Facebook is about to let us in on one of its darkest secrets: how many companies can track us outside of its service, thanks to its ad targeting tools.

Though it's been well known within the tech and advertising industry, it's not something the typical Facebook user has been given much visibility into. But that will soon change, thanks to the company's upcoming "clear history" tool.

The long-promised (and much delayed) feature will allow people to see which websites send your browsing activity to Facebook and remove that info from the company's servers. It is expected to launch in the next few months, according to Facebook.

But there's one group that may not be thrilled with the rollout: advertisers, who will have a more difficult time targeting Facebook users, once clear history becomes available, the company warned.

"When someone disconnects their off-Facebook activity, we won't use the data they clear for targeting," Facebook wrote in a blog post intended for advertisers. "This means that targeting options powered by Facebook's business tools, like the Facebook pixel, can't be used to reach someone with ads. This includes Custom Audiences built from visitors to websites or apps."

That may seem obvious -- many advertisers have long assumed "clear history" would impact their ability to target people -- but here Facebook is spelling out exactly what that means. Once someone clears their history, advertisers will no longer be able to use some of Facebook's most powerful advertising tools. For a company that has long touted its ability to help businesses reach the right people, this will be a huge change that will almost certainly negatively affect advertisers (assuming Facebook makes the tool accessible enough that it's adopted en masse).

Facebook, on its part, seems well aware that this won't be popular with advertisers, and repeatedly tried to position clear history as ultimately being "good for businesses."

"We believe that offering people greater transparency and control will ultimately have a positive, long-term effect on businesses using Facebook," the blog post says. In the short-term, however, things might get rocky.