These companies wouldn't have known about "PRISM", they would have known about their own individual subsystems used to tie into some NSA warrant system.

All these companies knew that NSA could get NSLs signed out, or even warrants issued by FISA before PRISM was made public, and they had all received such NSLs/warrants before they setup the infrastructure to handle those NSLs/warrants in a more automated fashion. NSA calls this infrastructure "PRISM", but each individual company wouldn't have been privy to it, because none of those companies would have a "need to know" (or a clearance) about the NSA's own special access programs.