[trotsky]

I believe that facebook won't allow current employees to trade on the secondary markets (though obviously there will be exceptions to that rule). I also believe that it's been years since facebook has been compensating with actual stock (since '07?), my understanding is that they take the form of restricted stock units that convert into stock once the company is public. This was to avoid breaking 500 shareholders at the time.

I doubt any of those are a primary concern - I think it's much more likely that since they broke 500 shareholders last year they've just assumed they would. With a calm, up market it's probably a better time to IPO than recent conditions. Being able to put $10B in the bank for a rainy day is nothing to sneeze at.