It seems like startups tend to move north once they get to the stage where they're not quite really startups anymore. Dropbox is a classic example. I don't know how easy it is to find space up there if you're not using a whole floor of an office building, though.
north? That doesn't sound like a general trend (I'd expect south), and doesn't match my understanding of Dropbox's moving pattern (153 Kerney to Market St. to near AT&T park.)
