MXit and AIM no longer exist. MXit as a company no longer exists and their website is gone. AOL terminated AIM in December 2017.
Yahoo! Messenger and ICQ changed protocols and Pidgin doesn’t have the developer resources to support these protocols ourselves. See the Plugins page for community-maintained plugins for these and other services.
Facebook terminated their XMPP support in April 2014. An alternative Facebook Messenger plugin was developed as a Google Summer of Code project. That plugin is currently in the Pidgin 3 source tree and has been backported to Pidgin 2. The backport can be found either here or on the Plugins page, but it is not maintained by Pidgin’s developers.
It is an incredible amount of work to implement and maintain support for a protocol. Pidgin’s developers are volunteers who work on Pidgin in their spare time. As spare time is a limited resource, Pidgin’s developers can maintain only a limited number of protocols.
That said, there are a number of people in the Pidgin community that have spent the time to develop plugins for additional services including Skype, Microsoft LCS/Skype for Business, and others. A list of known community-maintained plugins is available on the Plugins page.
If you are willing and able to implement and maintain a plugin for a protocol that you use, we welcome and encourage you to do so! The beauty of Pidgin and libpurple is its plugin system, allowing anyone to enhance and extend Pidgin.