A Facebook User is worth $300 per user according to Jeff Jarvis in a post titled Social value:

"…A Deutsche Bank analyst says that a newspaper reader in 2004 was worth $964 a year. Today, that’s $500. Facebook’s 50 million active users translates to $300 per at that valuation. And newspapers are shrinking while Facebook is growing by 200,000 new users a day. A day. And those users spend an average of 20 minutes each day inside the site vs. 41 minutes a month on newspaper sites, says DB."

I think Jarvis must have put in the wrong link, above, but as far as valuation of a user, I can believe that someone spending time, reading content 20 minutes a day would be worth at least $300 dollars..potentially (it terms of what they may purchase or signup for).

I like assigning a specific worth/value to an interaction/customer because it's easier to work with a known value than something totally unknown.   Is a Facebook user really worth $300… don't know. 

It reminds me of the act of painting - painting in areas that are empty is better done with a gray or neutral color - you know it'll be corrected later, but at least you've got the basis of some values to build on.