If you consider a platform cooperative to be a centralized and hosted system owned, operated, and governed by a cooperative, it is easy to tell what the cooperative owns.
If it's an open protocol, how does a cooperative own it?
A cooperative could form to create and maintain software that runs the protocol. SSB is starting more than one organization like that
Or cooperative could form to host sites that run the protocol, for example, social.coop. (part 1 of 2)