There's something slightly concerning about ceding responsibility for distributing the world's open-source projects from a family of strong independent repositories to a centralized platform owned by a tech giant.
It's possible many people have forgotten, or are to young to remember, how the ecosystem worked pre-Github. There was sourceforge, which wasn't quite the disaster it is today, but also not very good. But mostly I remember every project using different, often hand-rolled systems. PRs had to be sent in by mail. Every project had their own conventions of where to send patches, what formats to use, what additional information to provide etc.
Just try figuring out how to get a patch into Debian, which is still where most projects were ca. 2005. I won't wait.
I never contributed to OSS pre-Github. These days, I routinely send in a patch for smaller things I encounter a few times per week. Over time, I have also started becoming a more involved contributed to two projects. I doubt this would have happened without the flat learning curve that Github provides.
I wouldn't be surprised if both the number of contributors and total contributions to OSS have soared by a factor like 5x even above just the growth in OSS usage, and Github is the obvious reason for it.