Facebook has postponed its employees’ stock sale, perhaps indefinitely, the Wall Street Journal reports today. Facebook’s postponement is an understandable bow to market reality — and it prevents the company from setting an official valuation that the social networking site’s investors would consider too low. The market is pretty sure the company is worth less than the $15 billion Microsoft (s MSFT) bought into, or even the $4 billion internal valuation, but until a sale occurs, that value is theoretical (even if that theoretical value has very real business impacts). Maybe by the time its investors are required to officially asses the value of their investment in the social network, times will be flush, and they won’t have to write it down. But given the Microsoft money, I really doubt it.