I worked on Pidgin (Gaim, at the time), and was the original author of libpurple (now libgaim). Adium was a sister project that was originally completely different, but utilized some of our Protocol Plugins (aka prpls — basis for the name "libpurple"). It later utilized libgaim when it reached a certain level of maturity.

I don't know whether Adium eventually moved fully onto libgaim. I seem to recall it only used it selectively, but I stopped working on those projects years ago.

Been very happy to Pidgin's maintenance resume. Gary Kramlich's been doing a fantastic job reviving it, and I'm hoping the same will be done for Adium in time. We really need a modern multi-chat/IM client for the modern era.