Facebook is expected to go public on May 17th. That means the 6 month lock up for investors and employees would end around November 17th. This gives them the option to sell before the capital gains tax goes up due to the Bush capital gains tax cut expiring and the 3.8% tax on from the healthcare law taking effect in 2013. At least one member of Facebook's board has been telling people that the change in the capital gains tax rate is going to impact the markets, so this probably isn't a coincidence.
Hopefully the process goes smoothly and they don't have to push back the date much farther. During the holidays liquidity tends to dry up and employees and investors that decide to diversify their holdings closer to year end might move the market and give up in market impact what they are hoping to save in taxes.