Well, a billion is a lot of walking around money. If he just cashed out for no particular reason, it would point to a lack of faith in the growth prospects for Facebook. After all if you owned a billion dollars of a stock that you thought was a rocketship, wouldn't you leave it in and turn it into five or ten billion?
Of could it could be he wanted to spread his risk around, or pay his taxes.
Not if you don't care anything about the stock price or money. He just made 1.13 bln dollars for his company. If he invests it directly in fb related growth he no longer has to worry about what NASDAQ does. He's making fb independent of the media fluctuation or the fact that he has shareholders for all. The question becomes, what would you do if you you had a successful company and you were just given 1.13 bln dollars without any strings?
This is just a thought experiment, but I think fb brilliantly played wall-street and now has enough money to build an infrastructure. Internet advertising isn't creative, so what will the "next gen fb" look like? And is it worth investing in, speculating that it means nothing to them.
Of could it could be he wanted to spread his risk around, or pay his taxes.