We have over 700 users on https://lab.civicrm.org (a FOSS CRM for non-profits). While bits of the project still rely on Jenkins and Github, having most of our community on Gitlab really changes the dynamics of the project.

At first people were worried about losing contributors because "everyone is on Github", but increasingly it's the other way around. We have a lot of accidental techies who start issue tracking, testing patches, etc, and so it's nice to have a clear project structure.