@geek,
I'm with you on the Facebook front, don't get it myself, but each to his own.
As to usage, two options, get the app from your favourite App Store, or click the Messenger icon in Facebook.
Then it's just like any IM app, you type your stuff and the other party responds.
One advantage is you can set up Groups where you can just send to particular people.
I think you'll find it pretty straight forward to pick up.