Happy birthday Dries! If I may, can I outline a potential web-based birthday present based on your wish?
With relation to your desire to know who’s subscribed and potentially reading your posts, I think there are a number of ways forward, and even better, ways that are within easy immediate reach using Drupal as well as many other CMSes using some simple web standards.
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.
As an example, I’ve created a follow post for you for which my site would have sent a Webmention. Unfortunately at the time, your site didn’t support receiving it, so you would have missed out on it unless you support older legacy specs like pingback, trackback, or refback.
I also created a larger related Following page of people and sites I’m subscribed to which also lists you, so you would have received another notification from it if you supported Webmention.
I’m unaware of anyone actually displaying these notifications on their website (yet!), though I’ve got some infrastructure on my own site to create a “Followed by” page which will store and show these follows or subscriptions. At present, they’re simply stored in my back end.
As for Rachel’s request, this too is also possible with “read” webmentions. I maintain a specific linkblog feed (RSS) with all of the online material I read. All of those posts send notifications to the linked sites. While it’s not widely supported by other platforms yet, there are a few which do, so that online publications can better delineate and display the difference between likes, bookmarks, reads, etc. There’s at least one online newspaper among 800+WordPress websites which support this functionality. I suspect that with swentel’s Drupal module and some code for supporting the proper microformats, this is a quick reality in the Drupal space as well. Because the functionality is built on basic web standards, it’s possible for any CMS to support them. All that’s left is to ramp up adoption.
A quick note on Microsub and feed readers
Dave Winer in his reply to you linked to a post about showing likes on his site (presumably using the Twitter API) where he laments:
I know the Like icon doesn’t show up in your feed reader (maybe that can change)
Interestingly, swentel’s module also supports Microsub, so that reader clients will allow one to like (bookmark, or reply to) posts directly within readers which will then send Micropub requests to one’s website to post them as well as to potentially send Webmention notifications. These pieces help to close the circle of posting, reading, and easily interacting on the open web the way closed silos like Facebook, Twitter, et al. allow.