I guess such communities exist, but it looks weird to me. It shouldn't be like that. I mean, your project's community should consist of people who use or are interested in using your code. Whether it be hosted on github, launchpad, your own yourproject.com or another server capable of hosting the project. Hosting it is technical details; people should come to the project for what it is, not for where it is hosted at. Therefore the community shouldn't be "github community" or "sourceforge community" or whatever. Or did you mean communities such as "git users", "cvs users" and so on? I think that's still the wrong way to think of it. It sounds like a religious cult formed around specific tools.

They're just tools. I shouldn't need to ascribe to "vim community" hosted on "vimhub" even if I wrote my software using vim (I don't).

Sure, but when you set up the project, wherever you host it, you're going to have to choose an SCM before you have any community. If you want to provide the least friction for potential contributors, you'll want to to use the SCM most commonly used today in open source, namely Git.

Add'l note: Other hosts, BitBucket, Google Code, etc. also offer Git hosting. So, yes, choose whatever host you want, but choose and SCM that will make it easy for folks to contribute.