There's a lot of instant messaging services, and they've served millions of users and have marked their purpose. Some of them have been killed off as of now, and some of them still remain online. This is an article on some popular IM's and where they stand as of now.
AIM went into service in 1997 and was closed in 2017. The project AIM Phoenix was set to be a private server to put AIM back into operations. With it's recent news attention, the server could risk taking a strike from Verizon/Oath.
MSN Messenger was introduced in 1999 and was closed in 2014. Like AIM, the project Escargot MSN is set to bring MSN Messenger back to operations with it's own private server, and is currently pushing support to WLM 14 (2009).
Yahoo! Messenger was introduced in 1998 with closure being shortly in July 2018. A project by the same person behind AIM Phoenix is being made, sending Yahoo! Messenger back into operations. It currently only supports YMSG 6.0 and 5.5.
Pidgin is an multi-protocol client much like Kopete. It can utilize XMPP, IRC, OSCAR, and others. Because of it's multi protocol support there's no doubt the program will still get heavy use.
Skype was shown in 2003, and still runs in operations today, despite being in a horrible state. While you can't use older versions of Skype, you can still run it if you have a patched version (which unfortunately only works if you have a Skype Name.).
Discord was shown in 2015 and is used worldwide today. It has a large user base and has the option of creating community servers. There's nothing much else to say about it.
This is the list of software I can currently think of right now.
AIM Phoenix - http://iwarg.ddns.net/aim/index.php
Escargot MSN - https://escargot.log1p.xyz/
Y! Forever -http://iwarg.ddns.net/ymsg/index.php
Pidgin - https://pidgin.im/
Skype - https://www.skype.com/en/
Patched Skype - https://drive.google.com/file/d/0B0QZbkUatQzsY1hjSmtDeFJpVHc/view
Discord - https://discordapp.com/