Chief financial officer David Wehner said in a statement: "We expect fourth quarter 2021 total revenue to be in a range of US$31.5 billion (A$42 billion) to $34 billion.
"Our outlook reflects the significant uncertainty we face in the fourth quarter in light of continued headwinds from Apple's iOS 14 changes, and macro-economic and COVID-related factors.
"In addition, we expect non-ads revenue to be down year-over-year in the fourth quarter as we lap the strong launch of Quest 2 during last year's holiday shopping season."
The company said: "A new app tracking section in Settings lets you easily see which of your apps have been given permission to track you, so you can change your preferences and disable apps from asking in the future.
"Starting in early 2021, iOS 14 and iPadOS 14 or later will require developers to get your permission before tracking your activity across other companies’ apps and websites for ads or data brokers."
A few days after this announcement, Facebook said it would make some technology tweaks to cope with these changes.
For the third quarter, Facebook reported revenue of US$29 billion, an increase of 35% year-on-year. Profits rose to US$9.1 billion from US$7.8 billion year-on-year, an increase of 17%.
Wehner also said the company would break out Facebook Reality Labs as a separate reporting segment from the next quarter onwards.
"Under this reporting structure, we will provide revenue and operating profit for two segments: the first segment, Family of Apps, will include Facebook, Instagram, Messenger, WhatsApp and other services," he said.
"The second segment, Facebook Reality Labs, will include augmented and virtual reality related hardware, software and content. We expect our investment in Facebook Reality Labs to reduce our overall operating profit in 2021 by approximately US$10 billion."