I think the real goal with moving to a platform such as github is reducing the entry barriers for new contributors. As it has been mentioned already, almost all projects who moved from SVN with "send patches" contribution system, to Git/Hg with Pull Requests have got non-negligible growth in contributions.
I'm confused, maybe it's your use of a double negative (non-negligible), but that sounds contradictory? Are you saying that projects that have moved from SVN to git have had no growth or good growth in contributions?
Ged.