Perhaps in the vein of what you might be looking for, I’ve got a multi-user site built on WithKnown. It functions much like a stand-alone-Facebook-like service where users have their own accounts and can interact with each other on the service. It also has an OAuth server which allows users to use their own websites to log in and be able to post or syndicate content from their own websites into it, that way they have a choice of owning all of the content they post to it or not.
Note: this particular test site is meant more for folks to do quick test drives of the Known platform rather than serving as a platform in the way you’re describing. As an example of what you may be looking for though, here’s an original post on my own website (note the “also on” link at the bottom) and here’s the copy that was syndicated into the separate “community service” on an entirely different domain.
I suspect you could use other sites/services like WordPress to do something like this as well.