Lots of comments here about what this means for CircleCI. Found it interesting that CircleCI's CEO has a testimonial in the "What our community is saying" at https://github.com/features/actions:

“CircleCI has been building a CI/CD platform since 2011, and GitHub has been a great partner. GitHub Actions is further validation that CI/CD is critical for the success of every software team. We believe that developers thrive in open, connected ecosystems, and we look forward to working with GitHub to lead the evolution of CI/CD.” Jim Rose CEO of CircleCI