By Tom Taulli

Usually, CEO interviews mean little. But in the case of Facebook (FB), it seemed to be critical. Since Mark Zuckerberg gave an interview at the Disrupt conference a couple weeks ago, the stock's performance has been torrid, with gain of about 18%. Have investors gotten overeager?

That's the take of a recent piece in Barron's. It points out that Facebook’s valuation is at an unjustified premium. After all, the 2012 estimated price-to-earnings ratio is a hefty 47. Consider that Google (GOOG) and Apple (AAPL) have a multiple of 16x.

Based on this, Barron’s points out that a reasonable valuation for Facebook is $15. This is would be about 24 times the estimated 2013 profits.

Yet even this could prove to be too optimistic. Facebook’s revenues have been decelerating in 2012 because of the rapid shift to mobile. It’s true that the company is ramping up its monetization efforts -- such as with its Sponsored Stories (which are ads based on user comments) -- but it could take a while to get critical mass.

But there’s another big issue: the restricted stock overhang. To snag top-notch engineers, Facebook has had to issue huge amounts of its equity as compensation. However, this has two adverse effects. First, it results in a charge to earnings. According to Barron’s, this could mean that Facebook will trade at 35x earnings (assuming a stock price of $15).

What’s more, a decent amount of the stock will probably come onto the market, which could drag down the valuation. Hey, it’s not cheap to buy homes in Silicon Valley, right?

So on Oct. 29, there’ll be a lock-up expiration on 234 million shares. Then on Nov. 29, Facebook will allow 777 million additional shares to be sold. Already board member Peter Thiel has sold 80% of his holdings, while co-founder Dustin Moskovitz has sold about 5%.

Kind of scary? Actually, I still think Facebook has a bright future. With a user base of more than 950 million, the company will continue to be a major force. It should also figure out how to monetize mobile.

But for the next six months or so, investors need be cautious. The stock price could come under pressure because of the flood of shares hitting the market as well as the painful transition to mobile.

Tom Taulli runs the InvestorPlace blog IPOPlaybook, a site dedicated to the hottest news and rumors about initial public offerings. He also is the author of “All About Short Selling” and “All About Commodities.” Follow him on Twitter at @ttaulli. As of this writing, he did not hold a position in any of the aforementioned securities.

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