Personally I don't think there is one. Any such protocol would reduce these apps to a 'lowest common denominator' of features. And in the current climate, its

Personally I don't think there is one. Any such protocol would reduce these apps to a 'lowest common denominator' of features. And in the current climate, its features (not interoperability) that sells. Don't get me wrong, I'd love to see one. But I can't see it appearing until the business case demands it.