I imagine Facebook uses highly monitored virts that are spawned and destroyed constantly. I really wouldn't call them "servers" more like instances or staging hosts for containers. Root exploits aren't so serious in a ethereal state..
systemd is fine for single user systems and perhaps verts, but I would never deploy it on a server. The deprecation of cgroup v1 is a welcome change. It's come a long way, but it still has a very long way to go.