[andkon]

Y'know, Google has been doing this for a long time with AdWords: if your site doesn't have a high PageRank for the keywords you're targeting, you have to pay more.

I think the real problem here is the PR problem of charging for something that once was free. It wasn't a great idea, and the Tumblr example serves to illustrate how it could've been done differently.

[tomkarlo]

I'm not sure that's comparable; that's rewarding ads for being more relevant to the original search (or penalizing ads for being less relevant.) CPC services get paid for clicks but have views for inventory, so it makes sense they'd favor showing an add where the chance of a click is higher, raising the CPM.

On FB, you're getting charged just to show the post, so it's more of a CPM model.