I'm building a place review system.

Is there an advantage to using this protocol instead of a more application–specific one? Perhaps the shared identity?

A few things. Shared identity is one of them. Another is that applications can understand each other's data and mix them together, if they wish, or keep them separate, if they wish. An instagram-like client can read the posts made by the microblogging client and re-use posts that include images, for example. Same goes with the social graph: re-use one from another application, or create your own.