Earlier quoted context omitted.
> weirdly impersonal because it tells the recipient that you want them to know what you're up to, but you don't want to write a message just to them. Also, it would be very awkward to opt-out ("hey dude, um please don't send me your mass-email updates anymore.") You mean, like a post on FB ? Or Tweeting ? Readable for all ‘friends’ or ‘followers’ ? Who very likely are not really interested in your message ?
With FB, a person has opted in to being your friend. With a mass email list, whoever knows your email can send it. With FB, you go to the site to get basically this stream of opt-in announcements. With email, they are mixed with things that might require urgent action and direct personal messages and spam and what-not. Of course, I don't have FB on my phone and if I did it would be more intrusive.
And if for some reason you feel obligated to be someone's "friend" on Facebook, you can still opt out of seeing their updates. Not so with email, unless you write a filter that sends it straight to the trash.