Blogging, in it's many forms and on various sites, has existed since the late 90's, a form of one-to-many social media allowing for a single person (or a small group) to broadcast posts to an audience of readers. Generally focusing on a single subject (online diary, social media presence for a brand or product, general enthusiasm for a subject) and interactive in the sense that readers could comment on the entries, blogs have been a powerful tool in the world of fandom.
For the most part, blogging website LiveJournal was the hub for fandom discourse from it's inception in 1999. Not only did LJ allow users to maintain personal blogs, but it allowed them to subscribe to the posts of other users in an efficient and organized way. A "friends" page collected and displayed most recent journal entries from those followed blogs. Further, LJ offered a "community" option - users could create or join communities based on interests, and contribute content to those communities.
It is really here that text-based fandom discourse via social media becomes huge. What LJ offers that other blogging platforms lacked was a way to view the aggregate content of blogs a user is interested in (the friends page) and a way to organize as a community and contribute to a blog as a group. Fandom-based LJ Communities were a hub for fans - a great way to hear and share news, enjoy fan-creations like art and fan fiction, and to discuss the content in general. Also, multiple communities could be created for the same fandom, or for different parts of the same fandom (a community entirely for art, for instance, or a community dedicated to a single character), which led to content being "cross-posted" to multiple communities. LJ created a great platform for organized fan interaction - but because you could only find content if you actively searched for it, there wasn't a lot of controversy or criticism. Fans were looking to interact with other fans of the same material and LJ worked to actively bring fans together and to keep non-fans out.
For almost ten years, LJ reigned as king of the fandom blogging community. But in 2007, microblogging platform Tumblr appeared on the scene and in just a few short years hosted over ten million blogs. Unique features like different kinds of posts (text, image, audio, video, etc), an easy-to-use tag system, and ask box (allowing users to ask, anonymously or not, questions to the blog owner) made Tumblr appealing to users outside of fandom as well as in it.
However, fandom participation on Tumblr is huge. Where LJ had been a mostly text-based medium, Tumblr allowed for posts in all sorts of mediums. The tag system made searching for fandom-related content extremely easy, and while blogs dedicated to fandoms can't necessarily be joined, there are TONS of them. The main page of Tumblr functions similarly to the friends page on LJ, compiling posts of the blogs one follows. They're similar platforms with one massive difference - the reblog feature.
Tumblr allows for users to re-blog content from one another. This means that the content on someone's personal blog is not entirely original content, and that the content on your dashboard may not be content or a fandom that you are in. Further, Tumblr has a massive interest in social justice, and in both praising and critiquing work.
These factors combine to create an environment in which fandom discourse is not entirely positive, but it is extremely progressive. Issues of social justice - representation, racism, misogyny, white-washing, queerbaiting, etc - are constantly brought up in response to something as simple as a picture from the set, or as long and detailed as an essay. Even more interesting, sometimes users will comment on content they're not even fans of, content that just happened to be on their dash, providing fresh perspectives and outside views.
Because the content being created on tumblr is so progressive, so loud, so fast-paced, diverse, not necessarily positive, and extremely loud (in social media terms) - it is finally being recognized by content creators as a place where their content is not only going to be praised, but looked at critically.