I don't tend to see much writing of history on social media - in large part, social media (particularly Twitter) gets used to share writing, either from a blog or a journal; to livetweet speakers at conferences; to post job or fellowship openings or advertise PhD programs; to ask questions of other academics and students. The platforms that give you enough space to actually write - Facebook and Tumblr - don't seem to be widely used by historians in order to write history ... perhaps because non-historians with a tenuous grasp of history or historiography have made them into frustrating places? Right now posting an interesting photo with a "shocking" caption that may be completely untrue will get someone many likes and reposts, and trying to correct it will simply get you shouted down by the crowd. Even if you don't argue, it can be disheartening to see how much people like to celebrate inaccuracies that support their views.

Unless I'm misinterpreting your question - do blogs count as social media?