The 10b5-1 is actually bit fuzzy on whether you can cancel trading plan. SEC basically says that if you cancel the trade than you are not legally liable because

The 10b5-1 is actually bit fuzzy on whether you can cancel trading plan. SEC basically says that if you cancel the trade than you are not legally liable because no transaction actually occurred and so one cannot claim that you profited from insider trading. So in theory you can cancel the trading plan if you like. However if SEC feels that you cancelled only because of price drop then they can come after you for ALL your future trading plans or may even re-examine previous trades to see if you were trying to game the system. Therefore executives are advised by to cancel only under extraordinary circumstances. So I think Thiel had lot of legal pressure, although technically not an obligation, to sell as per his trading plan filing. He could have taken risk but it's probably not worth it for another half-a-billion of profit if you are already multi-billionaire and if you are already cultivating 1000X returns as it is. On the other hand it is surprising that he decided to sell most of his stock instead say only quarter or so. I would think Thiel as a long term investor in FB from what I've read in the book "Facebook Effect". Thiel is one of the few person that probably knows most upcoming plans and products in works and problems at FB. A typical strategy for investors like Thiel is to cash out in stages instead of dumping most stock at once like regular VCs do (10-25% each quarter, for example). Considering his level of insider knowledge this only means he did not expect FB stock going up at the time of trade filing in at least 1 to 2 year horizon. This is even more surprising because I keep thinking FB is going to come out with something like AdSense any day now and almost instantly double its revenues bringing stock back to $25-30. Obviously Thiel has calculated things differently.