I'll point out the blindingly obvious that nobody else can bring themselves to point out - future rounds have likely reduced that original 10% to a somewhat smaller number. There is also a pretty good chance that Thiel has taken some off the table, further reducing his stake.
Interesting - Facebook has had a pretty active secondary market - I've had a couple opportunities to invest, but couldn't afford the minimum buy in. What's your information that he hasn't taken some off the table? Seems like a reasonable move once it went north of $10 Billion.
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).
Still a great investment.