that a lot of this stock will hit the market in the next couple of months. Those who believe in market efficiency will argue that, because everyone has known about these lock-up releases, Facebook's stock price has already taken them into account. That's possible. But it's also possible that one or more of these releases will be accompanied by news (or inference) that big Facebook insiders are dumping almost every share they own, the way Facebook board member and early investor Peter Thiel did in the first lock-up release. That news clobbered the stock last time, and it could clobber it again this time. Facebook's Employee Stock Units Have Now Vested Today's lock-up release followed the vesting of Restricted Stock Units that most Facebook employees who joined the company before 2011 were given when they joined. Facebook converted these units into "Class B shares" and then withheld about 45% of the shares to pay the employees' income taxes (just the way the company would withhold taxes on cash compensation). These units converted into a total of 225 million shares and vested at a price of $23.21 per share. Thus, the vesting of stock units resulted in an immediate compensation payout of $5.2 billion to Facebook employees who joined the company before 2011, of which $2.3 billion was withheld. All of this stock is now available to be sold. Some of Facebook's senior executives received huge payouts in this conversion. These executives included: Sheryl Sandberg, COO: 34 million shares worth $790