So the general trend of Google search results projects that Tumblr is going to eclipse the general term 'Blog' before the end of 2012, cementing its meteoric rise to the most popular blogging platform.
But in the grand scheme of things, I find it difficult to classify Tumblr as a blogging platform.
Dictionary.com defines the term 'blog' as "A Web site on which an individual or group of users record opinions, information, etc. on a regular basis." This is, essentially, what Tumblr is and has the capability to be; but it isn't a space that promotes this ideology, and those that uphold it are few and far between. Instead, it's adopted many of the traits from social media, presenting a feed of the latest items from your friends, allowing you to reblog content, etc etc.
This means that rather than the aforementioned definition of recording an opinion, through well planned lexicon to make it sound furthermore elegant, you get the usage habits of the likes of Facebook and Twitter: the slow building of a personality through countless postings of photographs (especially with vintage filters), music, youtube clips and short pieces of text almost always complaining about something in some form.
While I feel that Tumblr has a place in the social sphere (we have one), allowing for a more dynamic array of content that Facebook doesn't allow you, and greater freedom beyond Twitter's textual constraints, I think the idea of it as a 'microblogging platform' (or blog) is somewhat far-fetched.
It's a social network, not a blog.