Ouch!Shares of Facebook dropped another four percent today, leaving the company's stock price just above $20 a share -- almost half where its shares debuted in a mid-May IPO. And let's not even talk about the $45 a share Facebook hit on its first day of trading. That seems like a distant memory. The news hasn't changed since Facebook's latest wave of selling, which began when with its lousy earnings report last Wednesday and continued with . Fear of mobile. Fear of a slowdown in user growth. And, in a sense, just fear. News that the some big boys have dumped the stock hasn't helped. The Wall Street Journal did some digging and found that Fidelity, one of Facebook's largest institutional investors, sold some 1.9 million shares across 21 of its mutual funds in June. CEO Mark Zuckerberg has often said that he's focused on the product, with the implication being that Wall Street can wait. That might be true, but Zuck can't be happy. Moreover, he has tried to have it both ways, using the promise of a powerful stock to build his business. Consider his pre-IPO purchase of Instagram, an acquisition that's still making its way through the regulatory process. When with Instagram founder and CEO Kevin Systrom, according to a WSJ report, he used the prospect of a soaring stock price to whittle down the price tag. Systrom wanted $2 billion, and Zuckerberg got him to accept $1 billion on the expectation that Facebook's rising stock price would get Systrom the price he wanted. In the end, they agreed on of Facebook. Systrom, who's still become a very wealthy man from this, is probably wishing he could turn back the clock. At today's closing price, the Instagram deal is valued at $761 million. Pretty cool, for sure, but hardly $1 billion cool. Of course, Zuck himself is getting hit by the selloff. His stake now sits at around $10.1 billion. That's an almost 32 percent drop in past six days of trading. Or, more to the point, he's lost some $4.7 billion, although of course that's all on paper.