Arguably, that's the whole point in owning stock in the company you own, control, and oversee. You can help make it go up and it's in your complete interest to help it become successful. You have no such control or incentive with other companies.
When you've got hundreds of millions of shares, you can sell tens of millions without losing ownership. At the same time you also insulate yourself from the risk of you or somebody who works for you fucking everything up.
Not just that but also macro variables that affect the performance of the equity markets as a whole... He is being prudent in diversifying a fraction of his exposure away not just of FB, but also from the asset class and perhaps even the currency / country.
