Facebook, Twitter, Zynga are hot companies and one day they will make hot IPOs. But what’s their value?

It’s often difficult to put a value on private companies because their financial data is private. But you can get some sense of their value by tracking the buying and selling of private company shares, and that’s whatSharesPost does.

The company announced its first value index today comprised of seven leading venture backed private companies.

The private market is where VC and other rich individuals can trade shares in venture-backed companies. You could call it the "smart money."

But you might be in for a surprise as to what the smart money values.

– Zynga, the maker of the Facebook game Farmville, has a $2.61 billion valuation. Twitter has a $1.44 billion valuation. The smart money sees $1.17 billion more value in Facebook games than it does in Twitter.

– According to the SharesPost Index, Facebook has a valuation of $11.52 billion. That’s a lot less than the $15 billion valuation it had in October, 2007 when Microsoft purchased a 1.6% share for $240 million.

It’s a 23 percent devaluation despite the massive growth at Facebook since Microsoft’s investment. And it hosts Zynga’s games, that has to add to its valuation.

Here is the complete list of the companies in the SharesPost Index and their current valuation:

Facebook $11.52 billion.

Zynga – $2.61 billion.

Twitter – $1.44 billion.

Linden Lab (Second Life) – $383 million.

LinkedIn – $1.3 billion.

Tesla Motors – $1.28 billion.

Serious Materials (Cleantech) – $227 million.

[Editor’s Note: Tom cross-posted this at Silicon Valley Watcher. –mrh]