Thanks for writing this Ben.
I totally agree that "product but open" rarely meets user needs. Especially when the need is a cohesive global community. That said these products do fulfill some users needs so I am happy to see their use, experimentation and focus.
And if one wants social to be decentralized and open I think a better approach would be to make decentralized technology so easy it's the simple default choice. Right now it isn't. Then, when the next social product would emerges it will have those principles baked in.