The trend took off in earnest when two things happened at once: Real estate prices fell in the wake of the financial crisis, and Twitter and Zynga gained prominence in San Francisco. They were able to expand here in part because of the softer real estate market. And their presence drove other companies to cluster around them.

"With Twitter and Zynga there, it's now believed possible to build a larger company in San Francisco," Josh Elman, a principal at Greylock Partners, said in an email interview.

Twitter announced last spring that it would expand into a hulking office space in the mid-Market district, a formerly troubled neighborhood across Market Street from City Hall that now seems headed for rejuvenation. In a highly public effort, Twitter tax from the city, which was endeavoring to revitalize an area with .

Later in 2011, Zynga outgrew its offices in sleepy Potrero Hill and moved into a massive headquarters at the southern edge of the South of Market district. Zynga at first leased part of the building, then, after its December 2011 IPO (initial public offering), bought the building.

With its cavernous lobby and exposed brick walls, Zynga's office exemplifies the type of warehouse and loft-like spaces that young tech companies are drawn to in San Francisco, particularly in the (SoMa) district.