idk it seems to me like tons of people on this server still use aim, it's a well-integrated part of many pb and celeb games.
people on tumblr seem to prefer skype (i know way more people there that have skype than aim) but ij has always seemed the opposite to me. i never see anyone advertising for psls with their skype contact for example. same with msn, yahoo, etc.
i do recommend gdocs though, they have the added bonus of being easier to tag into when you're somewhere you can't use aim like at school or work. or, if you have any irc experience, you might consider setting up an irc room for your game and including a simple how-to walkthrough for players who have never used irc before. i've been in games that have had private irc rooms and it's been a really great resource, provided that the mods at least are really diligent about being in there whenever they're available.