Are there any popular websites built on github.io?

Sometimes when starting new content websites, I think I could just use a GitHub page instead of a domain.

I mean, myproject.github.io is not worse than something like getmyproject.com, right?

But then I think: I never stumble across Github pages when I google for something. Maybe they are at a disadvantage compared to custom domains?

I know that one can use custom domains for Github pages. But I would like to discuss the effect of using just a github.io page.