MENLO PARK (CBS SF) – Facebook founder Mark Zuckerberg took to his on social network Monday afternoon to announce the formal opening of the company’s massive new building in Menlo Park.

Employees started moving into the new 430,000 square foot facility on 1 Facebook way Monday. According to Zuckerberg, the building includes “the largest open floor plan in the world – a single room that fits thousands of people.”

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“There are lots of small spaces where people can work together, and it’s easy for people to move around and collaborate with anyone here. On the roof is a 9-acre park with walking trails and many outdoor spaces to sit and work,” wrote Zuckerberg.

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The new building – which features a nine acre living roof – sits across the street from the company’s main headquarters building and is the 20th facility on the campus of the social network. Despite the fact that the structure was designed by renowned architect Frank Gehry, Zuckerberg says the building isn’t fancy.

“We want our space to feel like a work in progress. When you enter our buildings, we want you to feel how much left there is to be done in our mission to connect the world.”

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Zuckerberg said he would provide more photos once the staff is fully unpacked.