However they are a serious party, they're a highly motivated and active movement with significant influence in the Labour Party grass roots. They're sort of our equivalent of Antifa in that they are a significant organising presence in lefty street protests, but much more politically active and spend a lot of time in conferences and action committees wondering what Capitalism ever did for us.
Importantly, they are in no way shape or form a terrorist or criminal organisation. I would not be at all surprised if some ultra-violent or would-be Red Army Faction types joined or associated with them, but that's not at all who the SWP are as an organisation. They're the well meaning, impressionable white middle class girl next door with a class guilt complex and a side shave that's been going to BLM protests.
If they are getting banned by Facebook something has gone horribly wrong. Quite possibly some of them got a bit excited and posted things they shouldn't, so I'm not saying this is for sure Facebook's fault, but they're not about to murder any bobbies in Parliament Square.