Via Google Blogoscoped I read Robert Scoble's diatribe about how Gmail wasn't scalable, using Orkut and an out of context quote about invitation-only services taken when Gmail was 10 days old. In the post Scoble, who is normally far smarter than this, claims Gmail is less valuable to advertisers than Hotmail because Hotmail has 100 million active users, and if audience size doesn't matter then why do we have Nielsen ratings?

The answer, of course, is that advertisers care about cost over revenue, and when they can't get that, they care about cost-per-click, and when they can't get that they care about cost-per-impression, and when they can't get that they care about cost-per-estimated-audience. Nielsen demographics give a coarse guess as to who is seeing your ad. In contrast, keyword-targeted online advertising gives you precise ROI data that lets you hone your advertising campaign better than blasting animated gifs to users chained to their antequated, undersized and visually frenetic Hotmail accounts.

I was going to explain this in Scoble's comments, but lo and behold the rest of the Internet beat me to it. And people say I whine. (Well, I do, but that's not the point.)