Advertisers are worried that Facebook is about to become a data desert as it deliberately dries up a once easy source of rich data on its users.

On August 15, Facebook will shut down a program called “partner categories,” which gave its advertisers a way to tap directly into the largest consumer data brokers within the social network. Those brokers included Axciom, Oracle Data Cloud, Epsilon and Experian, which have information on hundreds of millions of consumers, and can slice them into highly desirable target audiences for marketers.

With the program winding down, a casualty of the Cambridge Analytica affair and new regulation, advertisers are scrambling to figure out exactly what they will be losing. Sure, Facebook can find people who like cars, but it can’t identify a luxury car driver whose lease is up next week. Facebook can say who is a mom, but can’t break down every mom’s household income, at least not as reliably as these data partners.

“Once third-party data goes away, we’ll see whether Facebook continues to be as effective as it has been,” says one advertising agency executive. “There could be brands wondering if they should even continue to advertise on Facebook. They can still do some interesting targeting, like people interested in yachts and rich people things, but that isn’t as good as targeting people with household income above $150,000.”

The agency executive and others spoke on condition of anonymity in order to discuss details of Facebook’s data program they were not authorized to disclose.

Facebook is shuttering the program because it wants to remove itself from the data responsibility chain in light of the new General Data Protection Regulation in Europe and heightened scrutiny of its business after Cambridge Analytica, the developer accused of mishandling data on up to 87 million Facebook users.

Facebook is coordinating with agencies and advertisers so they can license data directly from the third-party providers and then import that data to keep doing their ad buys.

Phillip Huynh, group media director and paid social lead at 360i, says Facebook has worked closely with the agency in preparation of the data changes. Facebook has been able to say exactly what clients will be affected and what data each brand will have to find elsewhere.

“Facebook will still have some robust targeting segments,” Huynh says. “Really, this is an opportunity for brands to take advantage of their own first-party data. That is where they have the most power.”

But many agency and brand executives say Facebook has handled the closing of the data program poorly, and it hasn’t helped them prepare for what to expect.

Now that they are scrutinizing the data program, participants are also realizing that they didn’t always understand what they were getting.

One agency executive recently discovered that the data a client was using to target ads came from a third party, not Facebook as they had thought. The brand will now have to find out how to replace it, and how much it costs.

“Facebook was, in the most charitable term, very unclear as to sources of data,” the executive says.

A Facebook spokeswoman declined to discuss the data program or its end in detail for this article. “We’re working with advertisers and our goal is to seamlessly transition them to alternative targeting options,” she said in an email.

Facebook may not even be clear itself on how all data in the partner program was sourced or how much each brand spent in the program, according to an executive at a marketing technology firm.

“We wanted to understand how much of our clients’ spending would be impacted,” the marketing executive says. “That reporting wasn’t immediately available.”

An estimated $1 billion of ad spend has gone through the third-party partner program every year, according to a person familiar with the program. Facebook says it passes about 15 percent of spending in the program to its participating data providers, such as Axciom and Experian.

The partner program is just a small piece of the overall ad business at Facebook, which totaled $40 billion in ad sales last year. But the advertisers that rely on the program are some of its biggest – the automakers, financial services, consumer goods and entertainment sectors.

Most brands are not concerned about how Facebook charged for the program in the past, but they are focused on overcoming the data gap when it arrives in two weeks.

“The partner program was effective,” says another agency executive. “When you use smarter data, sure the price goes up, but the results are stronger.”