Yahoo never recovered from Google. (Who has?) This makes all of their side bets into creating a social media network out of Flickr, Tumblr starting with their purchase of EGroups ten or more years ago so interesting. They had enough stuff to make a critical mass of a social media platform but never had the vision to unify those disparate products into one single space.
My guess is that there were a layer of vice presidents who each wanted to keep their own fifedoms and years of low level resistance prevented the 'Okay, let's turn this all into a single experience for the user'. They had a broad demographic spread over their different products but failed to reach ignition.