Now, I've been hanging around tumblr for a while now, and I've had a Yahoo account since everyone was calling it yahell (google was a search engine). My first reaction to the news was "how long until the 'anyone who doesn't reblog this will be counted as inactive and have their account deleted because there are too many people on here!' messages start making the rounds?" (drink for the irony that Yahoo actually used a version of this excuse when it shut down Geocities).
I wonder if Yahoo will have the common sense to let tumblr be tumblr and not turn it into something else entirely. In my opinion fandom is a force to be reckoned with and I estimate the fandom part of tumblr is significant, and when you fuck with it it will leave. Look at Livejournal, the usage of which has dropped significantly in the last two years (which coincides with some widely unpopular changes, as well as the tumblr's rise in popularity). Said changes on LJ caused significant traffic increases for their open source competitor - Dreamwidth.
Also, if you're tumblr user, you know know Yahoo thinks your blog is worth about 10$.
This wanted to be a post about fandom and the social media sites it aggregates on, but I need to go watch Game of Thrones now.