Comcast can't do anything to no longer be public infrastructure. Rather, municipalities would have to rework their regulations to allow anyone that follows a defined process to install the same infrastructure that Comcast has.
Likewise, Facebook would become public infrastructure when the government puts regulations in place that make it impossible or prohibitive to start a new social media site.
How do patents interact with it? If facebook got a patent on some new VR technology that allowed face to face interaction at a distance, and didn't license it out, would that turn the relevant portion of facebook into public infrastructure? What if they licensed it out but only to other people who also won't serve people saying <x>?
Likewise, Facebook would become public infrastructure when the government puts regulations in place that make it impossible or prohibitive to start a new social media site.