The BSD/Illumos OSs are used quite frequently as the base OS for high end commercial/enterprise network, SAN, NAS etc. solutions. They are chosen for it's performance, stability and HA features.
Also Redit started as "some BSD boxes" The problem is, when a project scales up, at some moment you will need "commodity" sys admins, so it is easier to just go for linux.
Also as the project gets bigger, at some point somebody will come with the idea to move to linux.
yeah.