I have a lot of sympathy for your Scandinavian viewpoint. But it's noteworthy that this is in the UK, which sets a relatively low bar to proving libel. Some say much too low.
I don't know how it is in Scandinavia, but in the US it would almost certainly be difficult to prosecute this. A lot of harassment is simply legal unless you can prove that deliberately false things were said with the intent to harm people -- which is very hard to prove.
Not that these platforms should hold themselves to a legal standard. Indeed, I've said before that it's long past the time for people to be thinking of moderation as an afterthought. New social networks -- even just discussion boards -- need to think from the beginning what kind of conversation they want. Otherwise, there's a very good chance it will be dominated by those who wish to hurt others, because the others will leave.
FB and Twitter are in a rare position where they own the natural monopoly (or, perhaps natural oligopoly) on general social media. You go there because your friends are there. You may want to leave, but you really can't because no other place can offer you the same connectivity even if they have a vastly improved feature set.
I don't really know what can legally be done to rein these two in. The UK is the lowest bar to set on prosecuting trolls, and this case was expensive and rare. It doesn't put trolls on warning because nobody else will have this person's resources. And it's harder everywhere else.
Even if you and I and all our friends left Twitter and FB for greener pastures, as long as they have that oligopoly, we'll still have to live with people using them to defame us. Everybody will know, even if we're not there.
I'm not actually worried about Trump. He was an exception. Let there be an exception for sitting Presidents; I don't care. He's still just one loud asshole. The problem is that anonymous jerks of all kinds can pursue defamation campaigns. They have to work harder to get traction, but they can also be more focused.
They, I think, are the real problem, and I don't really know what to do. I don't know what laws I'd pass even if I could. The vaunted power of the Internet has made it possible to be mean on a scale never before seen, and nothing I've seen in the past gives me a guide to that.