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When he moved Facebook to the Bay Area, Mark Zuckerberg deliberately set up shop in Palo Alto, not the self-involved hipstersphere of San Francisco. But he's been spending a lot of time in the City lately. Friday, a tipster spotted him in SoMa, near Twitter's headquarters. Kevin Rose, founder of Digg and lover of beer and women, met with Zuckerberg yesterday at the Samovar Tea Lounge. And Zuckerberg just added Twitter cofounder Evan Williams as a friend on Facebook.

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Here's something odd: Zuckerberg has had a Twitter account since at least May — mentioned in Rose's Twitter account of his teatime with Zuckerberg — but it seems mostly inactive. Is he just trying to fit in with the San Francisco crowd, or is there something more going on here? I'm reminded, for some reason, of the time Facebook PR director Brandee Barker friended her counterpart at Microsoft right before the two companies announced a big advertising-and-investment deal.