What's Facebook really worth? The fast-growing social network is adding to its 150 million users effortlessly. But revenues aren't growing as easily. And that has Mark Zuckerberg's company tied up in legal and financial knots.

Last summer, the company settled a dispute with a rival social network, ConnectU, that dates back to the founding of Facebook in CEO Mark Zuckerberg's Harvard dorm room. ConnectU's lawyers — whom the site's founders have since fired — revealed that the case was settled for $65 million in a newsletter bragging about their firm's accomplishments. And now the Associated Press has obtained a court filing which shows the exact breakdown of cash and stock Facebook used to settle the case: $20 million in cash, and 1,253,326 shares of Facebook stock.

That's no mere detail. ConnectU's ex-lawyers at Quinn Emanuel Urquhart Oliver & Hedges are pursuing legal action against ConnectU's founders — Divya Narendra and Olympic-rower twins Cameron and Tyler Winklevoss — to get them to pay $13 million. In other words, a 20 percent cut of the supposed $65 million settlement. But is the settlement really worth $65 million?

In October 2007, Microsoft paid $35.90 a share for $240 million in Facebook preferred stock, which only garnered it a 1.6 percent stake in the company. Preferred stock, the kind usually purchased by venture capitalists, have more rights and protections than common stock, which is the type owned by founders and issued to employees. And when a company is private, it's typical for preferred shares to have a higher value than common shares.

ConnectU's settlement was issued in common shares. And an appraisal Facebook conducted to value the shares it issued to employees valued the company at $3.7 billion, or $8.88 a share — making the stock part of ConnectU's payment only worth $11 million, and the total $31 million.

The uncertain value of Facebook's stock must be why ConnectU's ex-lawyers are in a dispute. If it had been paid in cash, why would they be arguing over how much the lawyers were owed? Instead of having $65 million, the Winklevosses and Narendra find themselves with $20 million in cash, a $13 million legal bill-and 1.25 million shares of Facebook that aren't worth nearly as much as they thought.

How little? An informal market for Facebook stock exists, though it's not publicly traded. Vulture investors are offering to buy shares for as little as $2.50 apiece. At that price, the company as a whole is worth $1.3 billion. That's less than Yahoo reportedly bid for the company in 2006.

And that's where Facebook could really get into trouble.

We hear that Facebook's salesforce had a series of panicked meetings last week as its salespeople tried to drum up more business. Google is actively working to steal advertisers away from the company, which has struggled to come up with new marketing products based on its users' relationships. (It does not help matters that Facebook COO Sheryl Sandberg, who worked in the Clinton Administration before joining Google to run customer service, has no real sales experience, as much as she likes to claim otherwise.) Meanwhile, Facebook continues to spend money as it attracts new users; every million users Facebook adds requires approximately $1 million in new servers.

So Facebook will probably need to raise money soon, and it will have to give up a far larger percentage of the company this time-a scenario its executives have dreaded, but which they have few ways to avoid. It will likely have to make whole Microsoft and other investors who bought in at a $15 billion valuation by issuing them new shares. That will further dilute the stake owned by employees, which will hurt morale and possibly lead to defections. If it loses key salespeople and engineers, Facebook will lose further momentum. The prospect of a death spiral is very real.

Right now, Zuckerberg's problem is averting disaster. After that, he can worry about how to get Facebook's value back up to $15 billion.