That is literally what ActivityPub and its predecessor OStatus are for: a protocol for declaring an identity and interacting with everyone else. The protocol is

That is literally what ActivityPub and its predecessor OStatus are for: a protocol for declaring an identity and interacting with everyone else. The protocol is heavily oriented towards exchanging small-ish messages asynchronously to a bunch of recipients or to a server, so it's mostly intended for microblogging, but there are already some other uses popping up (an instagram-like photo sharing system, a youtube-like video site where you can post/follow/comment through your account, a standard blogging platform...) All of those applications talk to each other in a way that is finally looking like it's got enough steam to hold.