I can confirm, this is the exact reason I made two copies of (shameless plug) the SpicaStack project earlier this month. GitHub's reach, network and stars help fellow devs and users discover our forks, while GitLab's exportability and amazing CI will let us actually work on it.

How do you keep the repos in sync?

You can setup git push to push to multiple repos.

git remote set-url --add origin ...

I use GitLab as the primary source, and mirror it to GitHub.

Or, I would have, if the builtin GitLab mirroring feature wasn't broken for pushing to GitHub.

I use multiple hosts to backup my repos: I host gitolite.org on a $5 VM and then have it configured to automatically push to gitlab and GitHub on every change.

My guess is some form of:

git checkout master

git pull github/master

git push gitlab/master

It's quite easy to set a git repo up to push to multiple upstreams. https://gist.github.com/bjmiller121/f93cd974ff709d2b968f also see https://stackoverflow.com/a/849960/308851

Absolutely - people now ask to see your Github repo as an integral part of your CV without even really considering if you might use another service.

And you don't exactly want to pitch to them how you don't have your code on Github because you use this other "fringe" service which even if they look at it they are going to find unfamiliar.

I now host 50/50 on Gitlab / Github because I'm not really affected by this, but I'm conscious of it.