While we may or may not be a bubble, its hard to pay much credence to statements like "there is no bubble!" by someone heavily invested in the potential bubble.
How many shares did Zuckenberg sell? He has not cashed out completely however he has certainly hedged his bets quite significantly.
Really? I would argue that somebody who has put millions of dollars behind his views has more authority than somebody who has put zero dollars behind them. He has a lot of money on the line. I'm pretty sure he's thought this through thoroughly.
No, he's an investor, who understands the concept of risk. He's made a bet that the value of companies he's invested in will increase, and it may or may not come to pass. His opinions on his investment however, maybe largely biased.