He's worth about 26 billion, I'm pretty sure that even if he cashes out 5 billion that is still plenty of incentive to deliver. He's waited 8 years, I don't consider that a "rush" to cash out.
If FB revenues continue to go up, then I'm wrong. But unless Facebook "finds" a novel business model I do not see revenue continuing to climb; I believe we are at the peak, and now begins the descent. If he waited eight years, then why not wait a little longer? I believe there is a reason now was the time. Time will tell.