iChat is not a service it's just an App. You need to plug in your own service into it. Like AIM, Google, Jabber.. etc. There are tons of people on those services.
There is also Adium http://adium.im/ Which is like an iChat, but has the ability to use more services. So you have no excuse to use Skype.
Again like I said above, "Skype if for people who are ignorant about computers." Which is why you're using Skype.
There are also tons of people who do NOT use those services, especially overseas, where MSN and Skype seem to be the preference. To be honest, I don't know a single person nationally or internationally who uses AIM, Google, Jabber, but that's just me.