““Cambridge or Boston. The same urban tech migration is playing out in San Francisco, Los Angeles, Seattle, and New York City as some of the hottest tech companies in those areas establish themselves in downtown offices. For example, Twitter Inc. is headquartered in the gritty Mid-Market neighborhood in San Francisco, and the crowdfunding site Kickstarter Inc. opened its headquarters in the Greenpoint section of Brooklyn, N.Y. “It didn’t surprise me that young people wanted to move back to the city, because cities are fun,’’ said Richard Florida, director of the Martin Prosperity Institute at the University of Toronto. “What really surprised me was this move among tech firms back to the cities.””