I'm still using livejournal.com social network nobody cares about today and when I was really into common lisp I decided to build a cli client to it. What makes it cool is that it's just markdown files locally and the client works almost like git, you can even pull and push posts. After I wrote it I was able to pull all my posts since 2007 or so from the service and have them locally as markdown files, any updates would be synced.
The other thing I've built mostly for my self is a notes taking service https://dabdab.org which allows to take notes the way I want it. What was cool about it was that I was able to almost reinvent django or rails but in go, so everything is fast, but still compile checks. From the product side I've managed to get to the same level of comfort one would get with github issues (markdown, image upload etc).
Both things have 1 user at the moment (me) do I think that counts :D