They allow recklessness (so you don't have to prove intent, you just have to prove that they saw the risk and did it anyway, which I suspect would definitely happen somewhere around the third FB account).
The guy was prosecuted under an objective standard - so basically, if most people think they were threats, they were threats. That's a very high standard for a criminal conviction.
Kagan makes some extremely reasonable points about how the distinction between hardcore political advocacy and incitement to crime is pretty damn narrow, so you have to insist on at least a recklessness standard. She's right, and so are the other ones who joined her.
I don't know that empty communities matter that much. If something's a niche interest, having a community at all seems more likely to encourage discussion. And asking people to start shouting into the void is a moderate-to-large ask. My local subreddit has 22K members, and took since 2011 to do that. It may be a little while before anyone else comes along, let alone comes along with something to talk about.
Also, while I have you here, there is no good reason to turn off ctrl-clicking opening a new tab.