[kbrobert]

No it does not. This is a myth that is always used by people who want to move to GitHub for other actual reasons that they cannot say in public.

I have first hand information from two projects (one of them large) that contribution, apart from drive-by doc fixes, does not increase.

Meaningful contribution decreases, because existing devs leave or become inactive because of GitHub.

A lot of projects moving to GitHub are in the Gervais-principle stage.