Facebook shares have tumbled more than seven per cent in after-hours trading after the company warned that revenue growth would slow this quarter, offsetting strong earnings that handily beat Wall Street estimates.
In a call with analysts, Facebook chief financial officer David Wehner said ad growth will 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 were down 7.7 per cent in after-hours trading on Wednesday, at $US117.35.
"They have reached the limit of the ad frequency on news feed, so they are going to have to find revenue growth from other areas like pricing, user engagement, user base growth," said Josh Olson, an analyst at Edward Jones.
However, he said investment in the business should benefit Facebook in the longer term.
"We have been down this road before with Facebook, they have invested something like this in mobile and we have seen it pay off. So we are looking at it as an opportunity," said Olson.
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 per cent of Facebook's total advertising revenue of $US6.82 billion in the third quarter that ended September 30, compared with 78 per cent 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 per cent rise in quarterly revenue, to $US7.01 billion, beating analysts' average estimate of $US6.92 billion, according to Thomson Reuters.
Excluding items, the company earned $US1.09 per share. On that basis, analysts had expected 97 US cents per share.
Facebook said about 1.79 billion people were using its site monthly as of September 30, up 16 per cent 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.
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