AD blocking apps worrying Facebook.

Ad blockers are a growing risk to Facebook's advertising empire — and it's only cash source, a new regulatory filing shows.

"Revenue generated from the display of ads on personal computers has been impacted by these technologies from time to time. As a result, these technologies have had an adverse effect on our financial results and, if such technologies continue to proliferate, in particular with respect to mobile platforms, our future financial results may be harmed," the filing says.

The company had cited the "impact of new technologies" as a threat in regulatory filings previously, but it was never called out in this much detail, as first spotted by Bloomberg.

The threat to its mobile advertising market comes directly from a September Apple software update that introduced ad-blocking apps to iOS 9. Ad blockers became some of the top-selling downloads in the app store, although some have since been removed over privacy concerns.

If ad blockers continue to take off, Facebook's river of advertising cash could quickly dry up.

Nearly 95% of the company's revenue per user comes from advertising, according to its Q3 earnings report, with only about 4% coming from payments and other fees. Of its $4.5 billion in advertising revenue, 78% came from mobile phones — a revenue source at risk of ad-blocking's popularity.