The average weekly wage in San Mateo County California, the home of Silicon Valley behemoths like Facebook, increased 107.3 percent in the last quarter of 2012, making it the richest place in America — even ahead of Manhattan — all because of a little IPO that happened in May 2012. Here’s how the thinking goes: Facebook went public in May 2012. In November — 181 days after the offering — those with vested stock options could cash in. Sure, CEO Mark Zuckerberg promised not do that. But if Peter Thiel, one of the earliest and biggest investors in the social network took out something like $1 billion, you can bet some less famous names did as well. In that quarter alone, the roughly 6,200 tech workers in the area collected total wages of $6.8 billion, notes the Wall Street Journal’s Scott Thurm. “That kind of a jump can only be explained by what’s happening with stock” Doug Henton, chief executive of Collaborative Economics told Thurm. Some of that likely also came from other tech IPOs that year, but Facebook’s was by far the largest.