I think the second reasons why FB and Twitter got so successful next to the network effect is that new posts don't require subject lines. When I still was on those platforms I often found myself just offloading random thoughts without thinking a lot about how to format or name that snippet.
When you write an Wordpress or most other blog engines your are required to think about a subject line. That alone is enough to not write anything in the first place at least for me. And I think many will agree with me on that.
This is also why notes on my website do not have a subject line. They are meant for quick and easy publishing.