This might be the first good move Yahoo has made since Mayer took over. I can't think of a better example of a company that needs to shed worthless baggage and focus on high value propositions. And I am gobsmacked Yahoo has not at least made an attempt at a real social network. They have hundreds of millions of users, at the point they are in giving all their users an automatic account on Yahoo Social might be a dick…
They've already got Tumblr; they just need to... do something, anything at all with it, rather than just letting it sit there. Integrate Yahoo's various news properties into the feed, and vice-versa. Make all of the commenting on every Yahoo property into gradual-engagement reblogs on a Tumblr blog you're not even realizing you're creating as you're creating it. Etc.
If they get it right,, I agree it could be a big deal, but the potential for totally destroying it by accident is great.