Is Tumblr a blog? It's growing like crazy. OTH, Posterous growth as a blogging platform seems to have peaked, which may explain their pivot to Groups, which is

Is Tumblr a blog? It's growing like crazy. OTH, Posterous growth as a blogging platform seems to have peaked, which may explain their pivot to Groups, which is a social networking and media sharing play, the heart of Facebook and instagr.am.