Facebook left its disjointed cluster of offices in downtown Palo Alto, earlier this year, and moved across town, consolidating into a big old former Hewlett-Packard manufacturing building next to Stanford University. While it has made the 150,000 square-foot monstrosity into an elegant location for its headquarters, the space is still not enough.

“Facebook recently leased 265,000 square feet of office space nearby on Page Mill Road, formerly occupied by Beckman Coulter, a medical diagnostics company,” according to the San Jose Mercury News. That’s a substantial increase in square footage. Why?

The new building is already nearly full. Facebook had nearly 900 employees when it moved earlier this year. Now, it has more than 1,000, according to the report, and plans to increase its head count by 50% in 2010.

Some operations will begin shifting to the new facility in the first part of the coming year.

For Silicon Valley historians, the expansion will be rich in symbolism. Hewlett-Packard was one of the first big technology companies to emerge in the region, and has helped define the valley as the world center of tech innovation.

[Map via the San Jose Mercury News; Facebook office image via ArchDaily.]