Yahoo! some months back could barely stand on its feet. Now they are buying successful companies and the internet users have no idea on what they intend to do with them.
About their e-mail.. While their new interface product was in beta, it was an amazing classic e-mail interface that included an API and features left missing when it was released as the default mail application. In one day it was replaced by a boring, minimal feature set with a lacking positive experience in its whole. While Yahoo! is still one of the leaders in web based e-mail, I am certain Yahoo! can do to Tumblr what they did to e-mail in just one day.