What Facebook did was essentially to make blogging have no entry cost.
It did away with the expectation that a post had to be a piece of well-drafted prose of a certain length. One-liners are OK in Facebook.
It did away with the intimidating backend. You write your stuff in the same place where you see it.
It did away with promotion and discovery. Your stuff is beamed to everyone else as soon as you post it (just like you get everyone else’s stuff.)
After that, it was just a matter of reaching critical mass.
The problem is one of ownership and archiving. What you publish on Facebook is mostly lost to the ether. There’s no easy way to keep track of it all or export it. If someone wants to know your thoughts about the book that you read three years ago, it’s there, but not that easy to find.
If you don’t care for the stuff you write (or paint, or draw, or compose), Facebook is a great platform.
But why should anyone else care about it, if you don’t?