Myspace (x users for y dollars)
Youtube (x1 users for y1 dollars)
...
Therefore users are worth (z+z1+...)/(x+x1...) and we can use that as a metric to value other sites.
But ignored all the cases of sites with x<i> users worth zero dollars.
I would venture to guess that based on revenue Tweetie is more valuable that Twitter itself. Myspace, Youtube, Facebook, they all have pageviews.
Twitter was founded as an SMS service and won't ever be able to fully monetize the platform unless they close the API (which they won't).
If you have 70M+ users, it takes a small amount of ingenuity to make money, even in the worst case (perhaps they're all Linux users).
To them, twitter are just a backend message passing system. Monetizing a backend message system is orders of magnitude harder than monetizing a client which actually has eyeballs.