Between 2000-2003, I worked on components of the cross protocol IM backend used by many of the multi-protocol messengers back in the early to mid 2000s (eg: Adium, Trillian, Fire, Ayttm, and more). Each of the frontends had different ways to integrate. Trillian used 2 processes with TCP communication between them to get around the GPL licensed backend), Fire, Adium, Ayttm released everything under the GPL.
Eventually most clients moved to using libpurple as the backend (developed by the team behind Gaim), but the devs also started getting older, busier, and having other responsibilities outside of work. The only apps to survive were those that had a business model that allowed them to reuse open source code without having to release any of the code they developed themselves.
I personally stopped working on instant messaging in April 2004, the night before I became a Yahoo employee, though I continued blogging and doing conference talks about the experience:
- https://tech.bluesmoon.info/2004/09/fallback-messaging.html
- https://tech.bluesmoon.info/2011/05/story-of-george-ayttms-m...
- https://tech.bluesmoon.info/2010/11/stream-of-collaboration-...