An email (whether direct or via a mailing list) is an interruption (at least to me), whereas a Facebook post just drops off the bottom of the page if I ignore it.
EDIT: Sorry, that was poorly worded.
The discovery side of it is more like an RSS reader (or maybe a group email). But the key distinction (for me at least) is that I can be totally passive - I don't even need to mark things as read - or I can engage.
10+ years ago I knew a bunch of people who had personal blogs, but of course most of them were filled with "sorry for the lack of posts recently" type posts.
I'm not on Facebook, but I understand why it's popular. The more people who might be reading, the more reason there is to post stuff. So it makes sense that everyone winds up in the same network.