In an effort to keep from accidentally tripping SEC reporting requirements, social media company Twitter has imposed an unusual blanket restriction prohibiting its stockholders—including employees—from selling more than 20 percent of their shares.

According to internal e-mails obtained by CNNMoney, Twitter imposed the controversial rule more than a year ago, but the announcement was never made public. The rule, which has created tension with employees, led to the resignation in August of Twitter's senior technical engineer, Evan Weaver.

In an e-mail sent to all employees last summer, CEO Dick Costolo cited the Securities and Exchange Commission's “500 shareholder rule” as the reason for the restriction on stock sales. Under the SEC rule, a company must begin making financial disclosures when it has more than 500 shareholders owning one class of equity shares. Even though companies are not required to issue a public offering at that point, many companies choose to do so once they start facing mandatory disclosures.

“We don't want to be public until we have very predictable quarterly earnings growth,” Costolo wrote in the e-mail. “We're not ready to be a public company for a couple years.”

What that means is that Twitter has to “artificially limit the supply of stock being sold,” Costolo wrote. “There is one reasonable way to do this: Let everybody with vested common stock sell only some fraction of their shares.”

It's understandable why Twitter stockholders would want to sell. According to its last fundraising efforts, the social media firm was valued at around $8 billion, CNNMoney reported. Furthermore, industry research firm eMarketer estimated Twitter's 2011 advertising revenue at just shy of $140 million.

“Folks continue to be worried about the company losing its best talent,” Janet Van Huysse, Twitter's vice president of human resources, wrote last month in a e-mail to employees summarizing the results of a recent staff survey.  

“In most cases, the comments name specific employees—most of whom left over the stock policy,” Van Huysse wrote in the internal e-mail obtained by CNNMoney. “You'll note that there are not necessarily solutions here—more so just anxiety that we lost so many ‘senior' people.”