See item #7 on this list of formerly popular dead-or-dying web sites: TypePad and Blogger. TypePad is where this blog is hosted. It does seem pretty obvious that Twitter and Facebook have grown at the expense of blogging. However, there is something amiss with the picture painted by this article. It says that "the streamlined blogging tool WordPress.com" has continued to grow.
I don't know where they got the "streamlined" idea. I've used all three of these services, and I don't think WordPress is any simpler or easier to use than TypePad. Blogger is probably simpler than either, overall, if you're content to use their standard layouts. So why would WordPress be doing better? Because, compared to TypePad, it's just as good, and it's free (or very cheap if you want more ability to customize their designs), whereas TypePad is not. And compared to Blogger, it's better, and also free.
But that doesn't explain why WP blogs would get more traffic--regardless of which platform is superior, I can't imagine that readers decide which blogs to read based on their platform. One doesn't sit down at the computer and go looking for WordPress blogs. So either WordPress is somehow doing a better job of publicizing blogs hosted there, or it has attracted bloggers who have attracted bigger audiences.