Option 3, by far. I use irc on a regular basis, and there are solid free programs for it on OS X/Linux and Windows platforms. On top of that there's auto-logging, browser-based programs as a fallback, and almost all have an automatic rejoin feature that actually works, unlike AIM's (where I will be locked out of group chats for hours at a time). The use of Topics would let you leave the initiative order stuck at the top of a room, so anyone who had questions could see it without having to scroll up...

Also, no ads, and people could hang around at non-mission times to chat about random things.