This was kind of a interesting topic, so I copied it over and extended it with some more steps which I remembered on the way into my blog: https://jeena.net/first-exposure-fediverse
I was checking what the definition of Fediverse is, and when I go after the one from the Wikipedia: https://en.wikipedia.org/wiki/Fediverse
Then it was when I implemented Pingback to my weblog so it looks like it was 2005.
Later around 2012, I was involved in the Tent protocol which was an alternative to OStatus and build a client for it. But because how bad their handled their community a year or so later I switched to:
Indieweb where I implemented a set of protocols and markup on my website to be interoperable with other websites which implement the same set of protocols and markup like microformats, micropub, webmention, etc. To this day I still am using my own website as the main publishing platform and automatically cross post every note and picture to Mastodon.
Otherwise I joined Mastodon with my own 1 person instance in 2019. And later PeerTube with my own instance for my family in 2020 and today I joined kbin.social just to test it before I decide if I should set up my own instance or not.
[edit:] I forgot that I set up my own Matrix server in 2020
How would one instance know that I set up a new instance without a central service?