I believe the battery issue is more related to the protocol through which they connect, some require a lot more frequent "pings" to the server to server know that you are still "online". The only one I leave on anymore is google talk and it has very little impact on the battery life, I have been contemplating turning on FB chat too as not many of my friends use google talk and I refuse to use AIM because it sucks battery life like a vampire sucks blood (by the quart).