I dont' have a dog in the hunt as I don't use any of these services but the few times I have looked at the main page of BS it seemed to have its fair share of negativity as well.
As
@Skyscraperfan mentioned, it seems ones choice of social network has now been reduced to another way to signal to ones tribe.
I see these networks as extensions of the public square and as such one is likely to have zero control over what one might see or hear. Sometimes you will agree with it, sometimes you won't, sometimes you will find it comical, sometimes you will be horrified. That's life.
Would you ban every person here at MR that speaks negatively of Apple, posts memes or makes a "crude" (but within the rules) joke as a way to only read positive things? I hope not.
For now. They all start that way, then the need to monetize rears its head.