That is not the issue with him obviously, but a few reasons why he would sell:
a) bring liquidity to the secondary market
b) realize some of his gains and book them for his funds figures for this year
c) hedge against facebook being at its peak. so eg. if he believes that there is a 10% chance facebook is at its peak now, he can then work out what % to sell to realize max gains based on risk assessment (Thiel is after all a hedge fund manager).
a) bring liquidity to the secondary market
b) realize some of his gains and book them for his funds figures for this year
c) hedge against facebook being at its peak. so eg. if he believes that there is a 10% chance facebook is at its peak now, he can then work out what % to sell to realize max gains based on risk assessment (Thiel is after all a hedge fund manager).