By all accounts, Facebook was not interested in hiring a marquee architect to make a structural statement. The company downplays corporate branding and discourages 'Facebook blue' on its existing 57-acre Menlo Park campus, at the southern edge of San Francisco Bay. The company of intense young programmers didn't want a space that would define them. 'We were used to being hermit crabs, moving into someone else's space and adapting it,' says John Tenanes, Facebook's vice president of global facilities and real estate. When Gehry first talked to Zuckerberg, a connection made by friends in high places, the company had already hired Gensler to extend its existing 57-acre campus with a set of four low-rise buildings accompanied by a parking garage. 'I think it was a learning curve for Mark,' says Gehry, who is quick to mention that Bilbao was built for $300 a square foot. 'He thought that to build for those prices, he had to have generically produced architecture. But it turned out that he didn't have to.'

Zuckerberg visited Gehry's own office, a renovated warehouse in Los Angeles, and liked the open daylit space, where architects could gather around large physical models. Gehry assured the young corporate head that he could design something to suit the company's budget and tight timeline, and won the commission.

For the campus extension'located on a narrow 22-acre site, adjacent to the older Facebook complex'the design team created a 1,500-foot-long structure with a continuous workspace on one floor. 'Every time we met with Mark at his office, more [partitions] would be torn down, so it was pretty clear how they liked to work,' says Craig Webb, design partner at Gehry Partners and the project designer. The steel structure sits on a reinforced-concrete podium supported by concrete columns, forming an at-grade, open-air parking garage. Like Gehry's office, the workplace has a lofty height of 25 feet. Based on feedback from city building officials, the structure, clad primarily in stucco and glass, is articulated into four primary masses to reduce its scale.

Nicknamed MPK20 (short for Menlo Park building 20), the elongated building is said to feature the world's largest open-plan office, yet it feels rather contained inside. Like many tech environments, it is organized into 'neighborhoods,' with clusters of desks interspersed with enclosed private conference rooms of various sizes. 'The dilemma was how to make this 400,000-square-foot space not feel like an automobile factory,' says Webb. 'We created a little city under the roof'we practiced indoor urbanism.'

This 'city' is designed without a regular office grid, giving it an ad-hoc, organic quality. A meandering path runs down each side of the building, with several cross-streets. The tall ceiling height also accommodates small mezzanine areas that offer more secluded work environments. 'This is the new paradigm for creative office space,' says Webb. Should the rumblings of discontent with open floor plans someday grow into a roar, the benefit of starting with a large open shell is that there is the flexibility to adapt.

Responding to Facebook's preference for raw, unfinished space'one of the company's tenets is that nothing is ever finished'the design team revealed everything overhead, including large-scale metal ductwork, custom skylight diffusers, and power and network cables that dangle down to the desks (for easy reconfiguring). Adding to the utilitarian ethos, exposed structural elements still have their construction markings on them, circulation routes are concrete (albeit polished), and conference rooms are simple boxlike gypsum board structures that could be torn down without much fuss. A bright and spacious cafeteria, anchoring one end of the building, offers the prime perk: three free meals a day. Here, everything from duck-confit pot pie to whole-wheat mango scones is made on-site in a large open kitchen. (Easy access to food is a big part of the design, with 14 coffee-dispensing micro-kitchens scattered throughout, two eateries under construction on the opposite end, and a grilled-cheese and potato bar planned for the roof.)

According to Gehry, Facebook did splurge a little on the rooftop park, which added about 10 percent to the overall cost. Initially proposed by the architect's team as a sustainability measure, the concept expanded into a full-fledged park. The '-mile loop with numerous branching paths accommodates 'walking meetings,' a popular way to conduct one-on-one confabs in Silicon Valley. When you are in the rooftop park, buffeted by the winds coming off the Bay, it feels like being on the crest of a hill. Buckling-restrained braces are in place to resist seismic forces. These supports also help manage the load of soil, plantings'including full-sized trees'and up to 5,000 people. Three glass-walled sunken patios break through the roofline and provide glimpses of the greenery from within.

While the architects were 'constantly trading amenities' to meet the budget, says Gehry, there are several distinct moments where the architecture asserts itself. The main visitors entrance, Lobby 4, is a triple-height space clad in stainless-steel panels. Within the sunlit volume, a sculptural staircase is clad in plywood: a humble material used to glorious effect.

Upstairs, the overall atmosphere is one of calm and quiet. 'A couple of hours after we moved in, people were hard at work,' says Tenanes. 'The space felt so natural to everyone.' One flight down, on the main office floor, an employee printout on one wall reads: 'Facebook Life Event: Saw Frank Gehry in a Frank Gehry building.