Peter Thiel's decision to sell almost all of his stake in Facebook Inc. so soon after company's initial public offering has few precedents in Silicon Valley, where venture capitalists typically hold shares longer.

Thiel, a Facebook director and its first outside investor, divested 72 percent of his remaining shares three months after it went public. Of the 40 biggest U.S. technology IPOs since the end of 2010, only Facebook and its underwriters let some backers sell so soon, with every other company adopting a so-called lock-up period about twice that long.

While venture capitalists commonly sell their stakes after helping startups reach the public markets, they usually whittle their holdings over a period of quarters or even years. That's to avoid flooding the market with too much new stock, which can drive down the shares, and to show continuing support for the company. Thiel's timing was particularly precarious, because Facebook was already down about 50 percent from the IPO.