http://adonomics.com/ - checking out the number of users, frequency of use and valuation of Facebook apps

Check it out, detals below:

What is Adonomics?

Adonomics™, formerly Appaholic, is your source for Facebook analytics. By providing a stock-market-style analysis of the Facebook platform we enable developers and investors to track application growth, activity, and valuation.

(as of Nov 25th 2007):

Facebook Facts

• There are 587,618,810 installs across 9,487 apps on Facebook with over 168,000 developers currently evaluating the platform.
• These applications were used 31,818,152 times in the last 24 hours and have a combined valuation of $504,543,658.
  
• Facebook has approximately 50 million Unique Active Users in the past 30 days and a valuation of $15 billion.
  
• This translates to $300 per active user.

http://adonomics.com/