I suspect you’ve been following Kristof De Jaeger’s work with the Drupal IndieWeb module which is now a release candidate. It will allow you to send and receive Webmentions (a W3C recommendation) which are simple notifications much the way they work on Twitter, Facebook, etc. I’ve written a bit about how they could be leveraged to accomplish several things in Webmentions: Enabling Better Communication on the Internet.
Not mentioned in that article for brevity is the ability to send notifications via Webmention when one makes follow or subscription posts.
This is a reminder that I really need to get onto my ‘Following Page’.