Comment by daveFNbuck

The cost to copy a repo over is low, but you lose all the social aspects. You don't copy over forks or stars. People who want to fork your repo and make pull requests need to sign up for a new service. Old links to your repo on GitHub won't work anymore.

I copied my personal projects to GitLab today and that was very easy to do but I'm not sure what to do about the GitHub version for the one that had a little popularity.