This is almost certainly going to be decided by the "reasonable person" test - and if you were on the jury it's going to have to be a higher bar than I, but I suspect there will be some offences we will both agree on.
My main point is not that we need to lock up everyone who makes a threat, but that we as a society will have to adjust our standards to the new normal.
Once upon a time every conversation was fleeting, every discussion in a pub or bar was ephemeral. Even Einstein and Dirac would walk home chatting without fear of being overhead. Then someone imagined it would be wonderful for the whole word to hear the erudite wisdom of those two geniuses of our age - and Facebook and Twitter and social media made it possible for every conversation in every bar to be captured and recorded and published - and we found out that Dirac and Einstein were just sledging each other and most other conversations globally were worse.
The new normal is that, like speeding, most evenings, conversations in most bars actually broke quite a lot of laws, from hate speech to sexual threats and basic politeness. And now the police can hear them as can everyone else - and discretion does not work on this scale - we either enforce the laws or change them.
That's a conversation for each judiciary- and likely to be either a balkanisation of the social media world, or a race to the top (we can all have twitter as long as we all behave to the standards of the highest / politest society. I am not sure where I stand on that.
Is it serious - hell yes. We are looking at a global technology with global benefits for all humankind - and if we want to communicate globally we need to agree what the standards for behaviour are on this virtual stage - from contract law to human rights and freedom of speech. We are inevitably going to build closer contacts - Brexit is a salutary lesson - and how we deal with freedom of speech online is just part of the jigsaw - but a telling part.