The currency we have right now on Twitter, Instagram and Facebook is reputation. There, it's manifested in the form of Followers. That's the very currency that Stack Overflow uses to grow and sustain.
It's "gamified". And the blogosphere needs that, too.
And in a way, blogs have that, but it's pretty much in the form of Reddit and HN and some other sites, where people get their karma fix by taking the time to read the blog post (upfront time investment), comment on it (risk losing karma) and then get their comment being liked (return on investment), which augment their karma (the reputation currency).
(Facebook does not have a Karma system like Reddit or HN has. But if it would, it would make the world a bit more like a certain black mirror episode of season 2.)
Why do you see coders put their reddit+HN profile links on their site? Because they want their reputation/karma/notoriety to follow them in real-life context, so it can be converted as real "notoriety currency". (people looking at their profile will say Gosh, this guy has 2000 points on SO and 4000 points on HN, he must be very smart or influent!).
So my guess is that the reason blogs still exist is because of sites like Reddit, HN, where people can get reputation (and not only knowledge) out of their reading. There's also something satisfying in the act of praising a good post with your friends. It's like watching a good movie and talking about how good it is with your friends.
And blogs also survived because it's still being viewed as THE most professional way of expressing one's opinions. It gives you "gravitas" by default, when you have your own blog, compared to having a Twitter / Facebook account like the "plebs" ;-) .
And you get the best of both worlds when you promote your blog posts on social networks.
So why not have a blog, then?