It will be the US company’s biggest engineering hub outside the US, with more than half of the team focusing on engineering.
Facebook first arrived in London 10 years ago, and the new addition will take its total UK workforce up to 2,300 by the end of 2018.
The 247,000 square foot office, which opened on Monday, is in Rathbone Place, just off bustling Oxford Street in central London.
Designed by the architect Frank Gehry, it includes seven floors and a new public square called Rathbone Square.
It will also include an “incubator space” for start-ups, called LDN_LAB, which will help kick-start fledgling businesses.
Some 400 construction jobs were created to build and fit out the offices from March to December.
Nicola Mendelsohn, vice president for Facebook Europe, Middle East and Asia, said the announcement showed that Facebook “is more committed than ever to the UK”.
She added: “The UK’s flourishing entrepreneurial ecosystem and international reputation for engineering excellence makes it one of the best places in the world to build a tech company.
“And we’ve built our company here - this country has been a huge part of Facebook’s story over the past decade, and I look forward to continuing our work to achieve our mission of bringing the world closer together.”
Chancellor Philip Hammond was one of the first to congratulate the company.
He said: “It’s a sign of confidence in our country that innovative companies like Facebook invest here, and it’s terrific news that they will be hiring 800 more highly skilled workers next year.”
London Mayor Sadiq Khan also welcomed the “fantastic” announcement, adding: “The launch of the company’s incubator is set to play a crucial role in attracting vital talent to London and will help to pave the way for the next generation of successful start-ups.”