Facebook moved into its new Frank Gehry-designed headquarters in Silicon Valley, with a rooftop park and "the largest open floor plan in the world."
Facebook co-founder Mark Zuckerberg insisted, however, that the building is "pretty simple and isn`t fancy."
"That`s on purpose," Zuckerberg said on his Facebook timeline.
"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."
Facebook`s new Menlo Park headquarters is one of several major projects by tech companies in the region.
Apple is building a new spaceship-like headquarters and Google has unveiled plans for a futuristic campus with wildlife and waterways.
Zuckerberg said Facebook`s goal was to create the "perfect engineering space" for its teams.
"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."
On the roof is a nine-acre (3.6-hectare) park "with walking trails and many outdoor spaces to sit and work," Zuckerberg wrote.
Facebook in 2012 hired the Canadian-born Gehry, known for the Guggenheim Museum in Bilbao, Spain and the Walt Disney Concert Hall in Los Angeles, to design the new building adjacent to its existing Menlo Park office, the former headquarters of Sun Microsystems.
Gehry, known for his deconstructive style and buildings that sometimes appear unfinished, also designed the Stata Center at the Massachusetts Institute of Technology and the Cinematheque Francaise in Paris.