Insiders at Facebook are selling stock in the social-networking giant, and the prices they’re getting for their shares suggest the sky-high valuation backers once placed on the company may prove unrealistic.

Just a few months ago, Facebook was widely viewed as the next Google (GOOG) in Silicon Valley. Microsoft (MSFT) bought a small preferred equity stake last October that the two companies said implied a valuation of $15 billion for all of Facebook. Shareholders in the still-private company appeared to be setting themselves up for a blockbuster initial public offering.

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The prices for Facebook shares in these transactions reflect a total valuation far lower than $15 billion. Albukerk, the founder and managing director of EB Exchange Funds, says that two current directors and one former executive recently contacted him about selling some of their stock for prices implying a $5 billion valuation. He also says that two investment firms have bought large chunks of Facebook stock at a valuation of about $3.75 billion. Hans Swildens, founder of Industry Ventures, a San Francisco firm that buys stock in private companies, says his firm has been talking with a growing number of Facebook employees. “There’s a lot of interest among people to sell shares,” says Swildens.