Facebook told advertisers in a blog post Wednesday it underreported ad performance on iPhones, citing privacy changes Apple made to its iOS operating system.

Facebook estimated it underreported web conversions on Apple’s iOS by about 15% in the third quarter, noting there’s a “broad range” for different advertisers.

“We believe that real world conversions, like sales and app installs, are higher than what is being reported for many advertisers,” Graham Mudd, vice president of product marketing at Facebook, wrote in a blog post.

Facebook shares fell about 4% on Monday.

Facebook CFO David Wehner warned of the potential effect of the iOS changes on the company’s July earnings call, saying he expected to see a greater impact from the changes in the third quarter. Apple’s update made it so that iPhone and iPad users would have to choose to opt in to being tracked when first launching an app. Specialized tracking helps Facebook and other apps deliver more personalized ads to users.

“We’re optimistic about our multi-year effort to develop new privacy-enhancing technologies that minimize the amount of personal information we process, while still allowing us to show personalized ads and measure their effectiveness,” Mudd wrote in Wednesday’s blog post.

In the meantime, Facebook recommended several actions advertisers could take to better understand their ad performance. Those include waiting longer to analyze data to allow for delays and considering using additional measurement tools.

Facebook said it’s already working to improve ad performance measurement, including by accelerating investments in reporting gaps ahead of the holiday season.

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WATCH: Facebook battles Apple over user privacy features in iOS update