Thanks to anonymous sources and the reporters who love them, we have a better picture this week of Silicon Valley's favorite guessing game: What is Facebook really worth?

TheWall Street Journal reported the social-networking giant is on track to make $2 billion this year in profit before interest, taxes, depreciation and amortization. Using published estimates from market-research firm eMarketer, the paper said Facebook is likely to rack up $4.05 billion in ad revenue this year, more than double 2010's take of $1.86 billion.

Whenever Facebook does go public, it would be subject to the basic laws of valuation -- once people know your real numbers, they value your company like they value other, similar companies. If you know how many times earnings -- the multiple -- the market pays for a few comparable companies we've featured below, it's not hard to do the rest.

Trouble is, the numbers for six selected companies point all over the place, implying Facebook valuations anywhere from less than $20 billion to $107 billion. But all of the comparables point to a lower number than the expert the Journal quoted on Facebook's value.

The bottom line: Of these six, only Salesforce.com makes a convincing case for the kinds of public trading price Facebook's fans think are reasonable. And there may be good reason for published reports that existing Facebook holders had trouble trying recently to sell shares at a $70 billion valuation.