Facebook doubled its profit in the second quarter thanks to a massive increase in advertising revenue, especially the average price of the ads it delivers to its nearly 3 billion users. But the company said it doesn't expect revenue to continue to grow at such a breakneck pace in the second half of the year.

Separately, Facebook said on Wednesday that it will make vaccines mandatory for employees in the US who work in offices. Exceptions will be made for medical and other reasons. Google announced a similar policy earlier in the day.

The Menlo Park, California-based company earned US$10.39 billion (NZ$14.93 billion), or US$3.61 per share, in the April-June period. That's up from US$5.18 billion, or US$1.80 per share, a year earlier.

Revenue jumped 56 per cent to US$28.58 billion from US$18.32 billion. Analysts, on average, were expecting earnings of US$3.04 per share and revenue of US$24.85 billion, according to a poll by FactSet.

Advertising revenue growth was driven by a 47 per cent year-over-year increase in the average price per ad and a 6 per cent increase in the number of ads shown to people. Facebook said it expects ad prices, not the amount of ads it delivers, to continue to drive growth.

The company said, as it has before, that it expects challenges in its ability to target ads this year — including regulatory pressure and Apple's privacy changes that make it harder for companies like Facebook to track people who can opt out of that form of surveillance.

Facebook had 2.9 billion monthly users as of June, up 7 per cent from a year earlier.

Shares fell US$11.77, or 3.2 per cent, to US$373.28 in after-hours trading. Earlier in the day, the stock hit an all-time high of US$377. 55 in anticipation of the results, so the decline wasn't unexpected.

- Associated Press