The first was Laconica which became Statusnet which became #Identica (2008).

Later (2016), after the merger of Statusnet and GNU Social (after Identica forked to stop using OStatus and went for Pump.io as protocol), several instances became famous: Quitter.se (and derivates like Quitter.es, Quitter.no, Quitter.is), Loadaverage.org, Gnusocial.de, micro.fragdev.com.

The main reason why Quitter.se in particular stood out in 2016 was that its admins and developers Hannes Mannerheim and @3mp0 created a UI resembling features of Twitter (which is why it was called Qvitter), with the goal to help Twitter users move to the Fediverse.

Today, with ActivityPub the main protocol and Mastodon the main platform software in the fediverse, gnusocial is a niche project. Gnusocial.net seems to be the largest one.