Remember this great article on the NYT (Zuck's most Valuable Friend)?

http://www.nytimes.com/2010/10/03/business/03face.html

    A few minutes later, Mr. Zuckerberg, Facebook’s co-
    founder and chief executive, walks into the company’s
    headquarters here, says a few hellos and heads to a 
    conference room where he and Ms. Sandberg huddle for 
    an hour. The two executives end the week the same 
    way, with a closed-door meeting on Friday afternoon. 
    They discuss products, strategy, deals, personnel — 
    and each other.

Given he has no office of his own, it makes sense that they hold their daily meeting in the conference room rather in his or her office.