Facebook employees have moved into their new Frank Gehry-designed Silicon Valley headquarters – a 40,000-square-metre office building with "the largest open floor plan in the world" and a huge rooftop park.

Company founder and CEO Mark Zuckerberg posted an announcement on his own Facebook page revealing that the social media giant's staff had begun occupying the new building on its Menlo Park campus in Palo Alto, California.

Known as MPK 20, the building was conceived by Canadian architect Frank Gehry, 86, as the world's largest open-plan office – approximately 2,800 employees will occupy one large room. 

"Our goal was to create the perfect engineering space for our teams to work together," said Zuckerberg.

"To do this, we designed the largest open floor plan in the world — a single room that fits thousands of people," he added. "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."

Zuckerberg has posted a single aerial photograph of the building on his Facebook page, with the promise of more "once we're fully unpacked".

Local users of Facebook's image-sharing platform Instagram were also invited to photograph the space yesterday. Their pictures show some of the artworks that have been created especially for the building by 15 local artists, as well as the 3.5-hectare rooftop park, which features a half-mile walking trail, a coffee stand and over 400 trees.

The building itself was designed and built in just three years, and comprises a relatively simple construction of metal, concrete and glass.

"The building itself is pretty simple and isn't fancy. That's on purpose," said Zuckerberg. "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."

Gehry – whose best-known projects include the Walt Disney Concert Hall in Los Angeles and the Guggenheim Museum in Bilbao – has described the completed building as a "remarkably human environment" with a "toughness" and "rawness".

"From the start, Mark wanted a space that was unassuming, matter-of-fact and cost effective," he said in a written statement. "He did not want it overly designed. It also had to be flexible to respond to the ever-changing nature of his business – one that facilitated collaboration and one that did not impose itself on their open and transparent culture."

Gehry was first appointed to the project in the summer of 2012, but was later asked to tone down his plans to make them more anonymous. In late 2013, he was also asked to design Facebook's offices in London and Dublin.