Just an hour before Facebook is set to announce its 'gmail killer', it now ranks as the third largest web company, with an estimated valuation of USD 41 billion which is greater than EBay.

According to Bloomberg, the social network's latest valuation, based on the selling price of shares on SecondMarket, an exchange for privately held companies, makes it the country's third-largest online business after Google (USD 192.9 billion) and Amazon (USD74.4 billion).

Since Facebook has yet to go public, its forecasted revenue and worth in the public market is all still theoretical. Facebook, which expects sales of at least USD 1.4 billion this year, has called the figure "fundamentally speculative."

Based on recent private-market transactions, Facebook has a total estimated value of USD 41 billion, exceeding eBay's USD 39.4 billion in market capitalisation.

The only two web companies worth more than Facebook are Amazon, at USD 74.4 billion and Google, Inc., at USD 192.9 billion.

Today's value of Facebook comes from an "off-the-record" conversation that Bloomberg BusinessWeek's Brian Womack had with someone who had access to recent transaction data on SecondMarket.com.

Now if Facebook were trading on an exchange that the public could access, the size of the company's total market capitalization might increase over the course of the trading day ‘ depending on the exact details that Mark Zuckerberg announces at today's 10 am PST press conference.

Facebook's value has been growing for almost three years now, but it seems to be accelerating recently.

While there's no doubt that Facebook is one of the biggest websites and one of the biggest companies of the moment, it's hard to know if the company's estimated valuation has anything to do with its actual worth.

And there is no way of knowing that until Facebook finally files for an initial public offering.

Based on what Facebook has been saying so far, that may be at least a couple of years from now.

The reason why Facebook is not jumping ahead and filing for an IPO is that, while the company is growing fast, its revenue is still not spectacular.

Facebook could bring in close to USD 1.5 billion or as much as USD 2 billion in revenue this year, depending on who's doing the estimates.

Facebook's users have increased by more than two-thirds since September 2009, when it had 300 million members.