Yahoo bought Tumblr last year for over a $1 billion. They have every right to use it as they see fit. However they will keep the best interest of shareholders in mind and the users of the site will come second. Getting used to the brave new world of ads on our dashboard is something we will have to do. I do predict at some point our actual blogs will have some sort of advertising on them.
I hope that Tumblr does not become another Facebook clone but we will have to wait and see. Some users are thinking about alternatives and considering options for the future. I am already ready for the future.