Here's a good chart tweeted by analyst Ian Maude.
He says, "Thinking about Twitter monetisation- ad revenue per user under 1/2 FB and 1/3 LinkedIn- lots of headroom for growth." And that's certainly one way to look at it.
Another way to look at it is how insane Google's ad revenue per user is compared to other ad-supported web companies. This is what happens when you have a magic money machine called search ads.
