I disagree.

I've been loosely involved in 2 open source projects which were previously hosted on self-hosted SVN repos.

After a move to Github, not only did the projects pick up new developers, but also gained a bit of visibility.

Github is something unique in my experience... it really is the "place to be" for developers wanting visibility (for any reason). A lot of folks use it as sort of a resume of skill and activeness in the community, and having public projects contributed to under their account serves to bolster that reputation.