Most of those cases turn out to be:
- ex-partner disputes involving harassment
- actual incitement to violence, like the hotel arson
And if you look at the actual convictions, first offense for most things is usually a suspended sentence. I'd be interested to see if you can find a case on bailii (no, not social media, actual court transcripts only) which matches:
- first offense custodial sentence
- one off post, not a pattern of harassment
- between strangers
- does not include even implied threats of violence
(Last one I can think of was the Robin Hood Airport one, which hinged on whether a joke threat to blow up an airport should have been taken seriously.)