I'd imagine the network effect is non-existent for private repos, but for public repos it's huge. And then there's the issue of switching between Gitlab and Github for private/public projects.
That's essentially it. GitHub has won its role as the (meta)repository of public record. I suspect many stick with it over its competitors for this very reason.
Exactly this. I generally begin my projects as private GitHub repos and then make them public once they have a MVP-quality demo of some sort.
I also make heavy use of GitHub's issues/milestones system, even on private repos, to keep myself organized. I'd probably use GitLab a lot more if I knew of an easy way to port issues and milestones over to GitHub when I decide to make a project public.
On a related note, can anyone recommend some good external issue tracking systems that integrates well with both GitLab and GitHub?