$40 million in Facebook shares were recently reshuffled in auctions on SecondMarket, leaving shares at $20.76 with 1.9 million shares traded. 2.5 billion shares were left outstanding (taking into account the 5-1 stock split earlier this year), placing Facebook's valuation at roughly $50 billion, according to All Facebook. This is the highest ever valuation for the private company, and frankly it's huge given that the social networking website is only six years old.

For the sake of comparison, the biggest business built on Facebook's platform, Zynga, was last reported to be valued at $5.51 billion. Two other new social network companies, Twitter and Groupon, were both recently looking for new funding at a mere $3 billion valuation each.

Given that Facebook has yet to go public, it's not fair to compare its valuation to the market caps for the biggest and most successful publicly traded technology companies. Still, we'll show you the numbers anyway: Yahoo ($21 billion), eBay ($39 billion), Amazon ($81 billion), Google ($186 billion), Microsoft ($217 billion) and Apple ($291 billion).

The $50 billion milestone, although unofficial because Facebook is private, still shows that many investors seem to have faith in the company's ambitions. Here's the letter reportedly sent out to SecondMarket auction participants:

To All Facebook Market Participants:

Thank you for your feedback and participation in last week's Facebook auction. As we had hoped, there was robust seller and buyer participation --- the auction fully cleared at a per share price of $20.76. In addition, there was overwhelmingly positive feedback on the auction process. As a result, we will be continuing to conduct these auctions for the foreseeable future. For this week, the process will remain reasonably similar, with slight tweaks to the timeline. The reserve price for this week will be $20.60, the same as last week.

If you own shares that you are eligible to sell and wish to participate as a seller, please complete the attached Seller Information Sheet and submit it to SecondMarket at fb@secondmarket.com by tonight, Monday, November 29th, at midnight EST.

If you would like to participate as a buyer, please complete the attached Buyer Information Sheet and submit it to SecondMarket at fb@secondmarket.com by Wednesday, December 1st at 12:00 PM EST.

Please see below for more detailed results on last week's auction and for this week's auction calendar:

11/22-11/26 Auction Results:

Total Number of Shares Offered for Sale: 1,896,265
Number of Shares Cleared in Auction: 1,896,265
Reserve Price: $20.60
Clearing Price: $20.76

11/29-12/3 Auction Timeline:
* Monday, November 29th at 10:00 AM EST - Auction process commences
* Monday, November 29th at 11:59 PM EST - Seller order forms due
* Tuesday, November 30th at 9:00 AM EST - Buyers informed of share quantity available
* Wednesday, December 1st at 12:00 PM EST - Buyer order forms due
* Wednesday, December 1st at 5:00 PM EST - Participants informed of auction results
* Wednesday, December 1st at 8:00 PM EST - Transaction documentation distributed to buyers and sellers
* Friday, December 3rd at 4:00 PM EST - Completed transaction documentation due from buyers and sellers
* Friday, December 3rd at 7:00 PM EST - Notice to be sent to Facebook, Inc.