I don't think so. The 2 specific situations you list are different. For child porn, it's illegal. So if Facebook blocks it, it's the government choosing what to censor, not Facebook.
And I think it's possible to prevent "harassing people with pictures of patterns of holes they find unnerving" without Facebook censoring content. Facebook has a block functionality, and the receiver could block the sender. As for the sender making multiple accounts, if Facebook blocks that, that's not censorship of content, that's enforcing a 1 account policy.
Facebook is also allowed to censor child porn in private messages, and that they do so is a good thing, yet it's still censorship.
Is it possible to use the word censorship without it being a rallying cry (which then seems to somehow ~bend reality)?
If Facebook wants to provide private messaging that’s actually so private they can’t know its content, that’s another story. But if they’re hosting messaging they know about, they’re entirely within their rights to decide what content is appropriate on their own resources.