Additionally, the notion that the necessity of making group decisions on a protocol must lead to stagnation is not guaranteed. There's plenty of space for Facebook, Google, LinkedIn and Twitter to form an an industry working group and make a protocol that meets their needs, and then throw it out and say "this is what we're using. Keep up or get left behind".
However, to more directly answer your concerns, there's nothing stopping Facebook from exposing basic functionality via an established and fairly firm protocol, and keeping advanced features as their competitive advantage. Textual Timeline posts, IM's and event RSVP's are good candidates for things to be exposed by the protocol. Facial tagging of images is a good example of something that a core social protocol could skip.
Social networks have been adding lots of features over the years to compete with each other. Realistically I think that fractured platforms would struggle to catch on because each provider would prioritize different features. Since social networking grows in utility superlinearly with the connectivity with your friends, features get less useful and the whole thing stagnates.
Basically: Microsoft Outlook is a social network of the kind you envision. Even Outlook for Mac versus Outlook for Windows is a sharp divide in terms of features! Do you think that people would use social networks if they looked like Outlook?