On Thursday, Facebook's (FB) shares dropped to their lowest level since the social media giant went public in May, as investors who were locked into holding 271 million shares became eligible to sell -- and sell they did. The stock, which debuted at $38 a share then briefly peaked at $45 a share, slid below $20 for the first time, closing at $19.87.

That was bad news for CEO and founder Mark Zuckerberg, whose fortune shrank by $600 million, to a still-hefty $10.2 billion. But while Zuckerberg can't sell his stock unit until the end of the year, any investor can now snap up a piece of Facebook, says Lee Brodie at CNBC. And several analysts say that "if you don't buy now, you'll kick yourself in days to come." 

At just below $20 a share, is it finally time to buy Facebook stock?

Buying now is a no-brainer: So "Facebook, the one-time darling of the social media investment community has fallen from grace," says Sherli Looi at Forbes. That's great: At $20, smart investors have a great opportunity to buy into a promising money-maker at a bargain price. "What is a fair value for Facebook? The answer can range between $30 or $55 a share," depending on whether the company captures "a measly 2% share of the global ads spend­ of $600 billion per year, or a more reasonable 5% share." Either way, the future looks bright for Facebook and its investors.

No. Facebook shares are still too pricey: Even with all the "carnage in Facebook shares" and new-found optimism among financial analysts, the company's stock still looks expensive, says Steven Russolillo at The Wall Street Journal. Facebook's revenue and new-member growth are slowing, costs are rising, and nobody is sure how -- or if --  Facebook is going to cash in on its billion users. Besides, its price-to-earning ratio is much higher than other tech titans, which alone suggests Facebook's "stock price remains too high relative to projected earnings, and could fall further."

If you buy now, know that it's high risk, high reward: The Facebook bears might be right, but "there's certainly a compelling reason to think about Facebook at or around $20 a share," says Matt Krantz at USA Today. Regular investors finally "have a shot at sweet revenge," since this is the rare time they could "make more on a hotly anticipated IPO than wealthy clients of investment banks" who bought at $38. Or not. The bottom line is that buying now is "an outsized and pricey risk," and most investors are better off with a diversified basket of large companies. "Speculators, though, cannot ignore a stock that's so down on its luck."

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