This page documents related and prior work.
The PingBack and TrackBack specifications solve a similar problem for tracking links between websites. However, they do not handle authentication or author identity (given proliferation of free hosting web sites) and so are susceptible to abuse.
The CommentBack specification had a similar motivation, but was XML-RPC based and relies on pre-registration of commenters at the source web site; Salmon avoids any need for pre-registration, logins, or permissions. I don't know of any deployments.