In a call with analysts, Facebook Chief Financial Officer David Wehner said ad growth would likely slow “meaningfully” due to limits on “ad load,” or the number of ads that Facebook can put in front of customers without alienating them.

He also said 2017 would be a year of aggressive investment that will see a substantial increase in expenses.

Facebook shares (FB)were down 6.9% in after-hours trading, at $118.45.

While the warning about the fourth quarter sent some investors running, by most metrics the company beat analysts’ expectations on torrid mobile ad growth.

Mobile ads accounted for 84% of Facebook’s total advertising revenue of $6.82 billion in the third quarter that ended Sept. 30, compared with 78% a year earlier.

The company is also reaping the benefits of a big push into video, both on Facebook itself and on the Instagram photo app.

Facebook reported a 55.8% rise in quarterly revenue, to $7.01 billion, beating analysts’ average estimate of $6.92 billion, according to Thomson Reuters I/B/E/S.

Excluding items, the company earned $1.09 per share. On that basis, analysts had expected 97 cents per share.

Facebook said about 1.79 billion people were using its site monthly as of Sept. 30, up 16% from a year earlier.

The strong numbers come as Facebook has struggled in recent months to combat allegations that it unfairly removes certain content on its service, and news in September that the company had for years overestimated how it calculates the average time users spend watching video.

But investors appear optimistic Facebook will continue to grow revenue through its aggressive expansion of mobile and video advertising.

More than 90% of Facebook’s users access the social network through mobile devices, and the company now boasts daily average mobile users of 1.09 billion, up 22% from last year.

Update: Nov. 2, 2016 (6 pm): This story was updated with additional information and the headline changed to reflect Facebook’s falling stock price.