In his place I'd promote bitcoins too I think that his investments in FB doesn't work

How do millionaires convert their bitcoins into dollars?

Exactly the same way as most people here do -- selling them through exchanges, or simply do it peer 2 peer. It's nothing special, really. In case they want to maintain a low profile, they will likely look for other wealthy individuals to exchange with, face to face. If they don't mind doing it officially, where they in all honesty pay their taxes, then it's just done through exchanges where they end up getting the fiat sent to their bank account.

It's great news that we are seeing more backers of bitcoins and cryptocurrency. This is good since they are influential people that could potentially create more awareness to the public that bitcoins are here and it is the future of finance. If more people like him come out and say that they use bitcoins then that would really boost prices more.

I was wondering why many millionaires and billionaires now are coming out and claiming they have big investments behind bitcoin and other crypto-currency? First they was  Mike Novogratz now Chamath Palihapitiya?  Are they hyping the already "hype" market? When the price of bitcoin was below $1000 you will not see anyone coming out telling I have millions in bitcoin as investment? I guess, those individual who has a lot of money is really intelligent when it comes to investment, and they saw the bitcoin is a safe haven for them to invest to for the longer term.