Facebook’s Seattle office was already its largest engineering center outside of the Menlo Park, Calif. homebase, and now the social giant is opening up a new building with room for 1,000 more people.

Teams have begun moving into a building called Arbor Blocks 300, a six-story building with 196,000 square feet of office space at 300 Eighth Ave. N. in Seattle’s South Lake Union neighborhood, the company said. The office, which sits in the heart of Amazon’s campus, is part of the new two-building Arbor Blocks complex that Facebook leased from the late Paul Allen’s Vulcan. The other building, a 189,000-square-foot structure, will open in the next few months with room for another 1,000 employees.

“This office will support Facebook’s growth in Seattle and help us attract people from the region’s tech community,” Vijaye Raji, head of Facebook’s Seattle office and the company’s vice president of entertainment said in a statement. “As we continue to grow here, we remain committed to being a good neighbor and having a positive impact on the community.”

Facebook said it has 3,000 people working in the Seattle area today, but the company’s real estate expansion adds capacity for thousands more. In February, the company said it had 2.7 million square feet of current and planned space, which includes Arbor Blocks and a growing campus in Microsoft’s backyard of Redmond, Wash. for Facebook Reality Labs, the virtual/augmented reality unit that also includes Oculus Research.

Using conventional office space ratios as an estimate, Facebook’s future capacity in the area could be somewhere between 13,500 to 18,000 people. However, much of the Oculus campus is for lab and research space, requiring more space per employee, so the capacity is probably somewhere closer to 10,000.

Facebook is growing all over the region, but much of its expansion has occurred in the South Lake Union neighborhood, known primarily as Amazon’s backyard. Google is also establishing a major presence there, setting up an intense competition for talent that can choose between three of the world’s most recognizable tech companies, all within walking distance of each other.

Engineers in the Seattle office work on a variety of projects, including infrastructure and machine learning and products like Messenger, Marketplace and Games. GeekWire recently got an inside look at how Facebook is refining its gaming division, with much of the work happening in Seattle.