As an EFnet admin: EFnet ;-) IRCnet is still around too with some very active chat channels (e.g. #worldchat). The networks are much smaller nowadays though.
Want to talk about a FLOSS project? Libera.Chat, OFTC, GIMPNet.
Want to just chat? Snoonet, Slashnet, Quakenet.
Want to just chat but want it to be technology focused? Darkscience, 2600net.
This is a shortlist of public networks I can recommend to a new/returning user; there are many smaller networks out there that you might discover organically with time.