A large portion of the people I know use some sort of Instant Messaging program. Most use AOL Instant Messenger, either via AOL's app or a third party client like iChat. Microsoft's MSN Messenger and Yahoo! Messenger make solid showings and there is always the occasional ICQ user. The time has come for something better.
The dirty little secret of all these "free" IM networks is that they are owned by a corporate entity. All of your chatting heads to a central server managed by people working for a company who's main motivation is profit. This is not a bad thing necessarily, but it means your best interests are not at heart. Read the license agreement for your service of choice some time. They are invariably terrifying, if for no other reason than there will be a clause that says, "We can change anything we want at any time without telling you."
I've long thought it would make more sense for IM to be based on open protocols with decentralized management. When you use email, you get an account on a specific server. That server is owned by your ISP or your employer, or maybe even yourself (like mikemchargue.com is for me). The chain of accountability is much shorter. There's also no central authority like there is with current IM networks. All email does not end up in one place where it can be reviewed, despite what conspiracy theorists would have you believe.
Jabber takes IM and makes it work a lot like email. I now have a Jabber ID, firstname.lastname@example.org, that is managed be me. Messages sent to others are more secure for a number of reasons, but mainly because messages do not pass through a central authority.
I highly encourage everyone to give Jabber a try.