San Francisco, United States -- Facebook on Thursday postponed workers' return to the office until early next year due to surging cases of the Delta variant of the coronavirus.

The leading social network set a new target of having employees back on its campuses in January, but promised to provide ample notice before they are required to show up.

"Data, not dates, is what drives our approach for returning to the office," a Facebook spokesperson said in response to an AFP inquiry.

"We continue to monitor the situation and work with experts to ensure our return to office plans prioritize everyone's safety."

For now, the data shows rising numbers of COVID cases based on the Delta variant, the spokesperson said.

Google, Facebook and Microsoft have said returning workers will need to be vaccinated against COVID-19.

Facebook is also requiring people in offices to wear masks, regardless of whether they have been vaccinated.

At Microsoft, the earliest date for fully reopening US facilities will be October 4, according to the computing giant based in the state of Washington.

E-commerce colossus Amazon confirmed that it is delaying employees' return to its corporate offices until January of next year instead of September as originally hoped.

Google this month made its campuses off-limits to unvaccinated employees and extend its global work-from-home option through October 18.

Unions and critics of mandates have spoken out against required vaccinations, citing personal freedom arguments.