For public forums it may be that People just prefer to be isolated from the other side by a barrier larger than a single platform and therefore a single overarching takedown policy can provide.

I think Facebook skirts it because it doesn't really have a public forum rather it has a large buffet of build your own forums that are not publicly visible from each users friends feed and a "let's solve for a feed of content likely as possible to keep said user on our site using as much history / personal information we can datamine about them, their k-nn's, and users in general"

For example "how voat turned out" seems to be ideal for a certain portion of internet users unwelcome by other sites and so perhaps they congregate there thus skewing the sample towards the way it turned out.