Seconding the fact that AIM still has a reasonable presence with IJ RPers, though very few people still use it for actual RP as opposed to talking about RP -- plotting, character development, etc.
Google Docs are IJ RP's current bread and butter. Some people still write things out on IJ's actual site, but over the past few years I've found that number seems to have decreased pretty significantly. Writing in Google Docs and chatting with Gchat is extremely popular right now.