FORTUNE — Last month I watched Peter Thiel publicly debate Google’s GOOG Eric Schmidt like an abused pit bull debates a labradoodle. This came on the heels of a much talked-about 60 Minutes appearance, in which Thiel expounded on his libertarian values and why many college diplomas aren’t worth the paper they’re printed on. In short, he’s a guy with lots of ideas who wants everyone to listen.

So you’d think he’d be happy to explain why he sold around 22 million shares of Facebook Fb stock last week, as soon as a lock-up period had expired. After all, he’s both the company’s first outside investor (save for Mark Zuckerberg’s father) and a board member. But you’d be wrong.

Thiel has suddenly gone quiet, refusing to answer questions from reporters. Both myself and a colleague reached out directly this week, but received no reply.

To be charitable, it’s possible that Thiel is stuck in an undisclosed location without Internet access. After all, it must take forever for Verizon forever to hook up an artificial island nation.

But the sales occurred one week ago today, and Thiel’s silence is deafening.

Did he sell because he thinks Facebook stock is appropriately priced at around $20 per share. Did he sell because he thinks $20 is a ceiling? Did he sell because of personal financial issues, or because of expected capital gains tax rate increases? Did he sell as part of some broader portfolio strategy? Did he sell on a dare?

None of this would necessarily matter if Thiel was just a large outside shareholder but, again, he’s on the Facebook board of directors. As such, he has both access to inside information and a fiduciary responsibility to the company’s other shareholders.

By selling so much stock last week at half of what the company was worth three months earlier, he seems to have enriched himself while simultaneously telling the market to look out below. How is that latter part in the best interest of other Facebook shareholders, many of whom also happen to be Facebook employees? How is it in the best interest of company morale when one of its leaders signals a lack of faith in its direction.

Again, perhaps Thiel could adequately resolve those contradictions by publicly explaining his decision. Or by stepping down from Facebook’s board. In the meantime, Thiel’s uncharacteristic reserve is hurting the company that helped make him a billionaire.

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