Just a few years ago, even the best internet companies would have found it hard to go public. With the global economy faltering, the word ‘IPO’ wasn’t on anybody’s mind.

Times are different today. The global economy is still a source of many concerns, but the stock markets are currently shrugging those concerns off and giving the top tech companies an opportunity to sell their shares to the public.

Last year, big names on the consumer internet, including LinkedIn, Groupon and Zynga, made their public debuts, and soon Facebook will join them in what is expected to be the biggest technology IPO ever.

But at least one consumer internet darling, Twitter, isn’t looking to go public anytime soon. In fact, it’s apparently so eager to prevent a public offering that it has been restricting sales of its shares on the private markets.

CNNMoney has the details:

…[Twitter] has slapped its shareholders with an unusual restriction: No one who holds stock can sell more than 20% of their shares.

The rule — which has been in place for more than a year, according to e-mails obtained by CNNMoney, but is being reported here for the first time — has caused dissent in Twitter’s ranks.

Not surprisingly, some Twitter shareholders aren’t too happy. According to CNNMoney, a number of employees have resigned as a result. One, a high-profile engineer, cited “policy disagreements” in his resignation, resulting in a response from Twitter CEO Dick Costolo explaining that Twitter is trying to avoid surpassing 500 shareholders so that it isn’t effectively forced to go public.

The “500 shareholder rule”, which forces companies to disclose certain information publicly once it exceeds 500 shareholders, is one of the big reasons Facebook is going public. Facebook, of course, has the financials to go public, but apparently Twitter isn’t as confident.

CNNMoney obtained Costolo’s email from last August, in which he states, “We don’t want to be public until we have very predictable quarterly earnings growth. We’re not ready to be a public company for a couple years.”

That obviously puts Twitter in a precarious position. More disappointed early employees unable to cash out more than 20% of their vested stock may decide not to stick around for an IPO, putting Twitter in a talent bind that could see the company’s ability to innovate decline just when it needs the innovation the most.

The big question is why so many Twitter employees apparently want to cash out in a big way. The obvious answer: the company’s valuation has soared in recent years and at least some employees, if not many of them, either believe their employer’s fortunes have peaked or are concerned that in a “couple years”, the IPO window may have long closed. Judging by the skepticism around even the Facebook IPO, and the global economic situation, they just might be right.