I'm no entrepreneur, but I really don't think sites like Tumblr or Reddit (or 4chan) should be VC $$ backed startups or bought for a billion dollars. I've never seen them be able to monetize past maybe their server costs.
In the age of pretty much unlimited ad inventory and user ad blindness your only hope to improve click-through is (a) to know your user and (b) be able to grab their attention in a consistent manner, preferably several times a day. Not sure how Tumblr executed on either, most of my visits there were logged-out referred through some other sites.
