I have no idea what Joe's option situation is, but supposing that he's cashing them in and dumping them in the secondary market it goes to show that Facebook is squandering the employee lock-in advantage of having pre-IPO stock. It's probably in Facebook's interest to shut down the secondary market, but I imagine zuck and co like being somewhat liquid for personal reasons.
If your goal is to have people doing creative, innovative work, then I would strongly disagree that there is any advantage whatsoever to be derived from making people feel "locked in." In practice, I have only seen this achieve the opposite effect -- disengaged people hanging around longer than they should and poisoning the atmosphere for passion-driven folks around them.
Joe's post made it quite clear that Facebook's management strategy is to provide freedom and autonomy, not handcuffs. This is one of the key reasons people love working there. The only thing Facebook is "squandering" is the opportunity to be experienced as a corporate financial prison.
The idea that any of this would be driven by Zuck's personal desire for liquidity is way off the mark.
Being able to work on small teams on interesting projects that reach 100s of millions of users and get paid reasonably well is sufficient incentive to work for Facebook I should think.
They may have already written off the employee lock-in effect and are taking their time going public just to avoid the disclosure and compliance hassles associated with it.
