my impression is that most people using social media treat the contents of any individual tweet/post as something that exists in complete isolation from any broader context.

I share your frustration with this and, at least in regards to things about current affairs/politics, I always like to investigate the source, in fact I think people have a duty to do so before they share tweets/posts etc. The amount of misinformation being spread around and posts being shared that are written by people who shouldn't have any kind of platform is a particular annoyance for me with tumblr at the moment.

Sorry, I don't mean to rant on your post. It sounds like leaving twitter was a really good decision for you, especially as it can be so hard to give up these kinds of sites.