its always interesting to see how revenues equate based on unique visitors. too often people are caught up in the ego of page views or impressions and miss out the one fundamental fact – uniques drive page views. if you have the uniques you are more likely to have people clicking on ads, buying stuff etc which transforms into value for you. you really cannot do much with page impressions because even if you have a page depth of 1000 per visit but only one unique visitor, the chance of getting a lot of advertisers scurrying for the space is limited.
anywhoo, below is a graph that looks at google, amazon, facebook, badu, mail.ru, ebay and the usual suspects in the web game. i dont see a microsoft or twitter number here, but here is a blog that i did a while back that covers that.