Pidgin is a multi protocol chat client. It works on Linux and PC, technically it also works on macs but it isn't as simple to install on OS X as other applications you might be used to. I would try Adium if you're using a Mac. The cool thing about using a multi protocol client like this is that you don't have to install three or four or more different programs in your computer, eg, yahoo, msn, aim, and so on. You also don't need to log in to websites that have chat enabled.
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Some websites allow you to connect to their chat without using their website. Pidgin allows you to connect to Aim, Bonjour, Gadu-Gadu, Google Talk, GroupWise, ICQ, IRC, MSN, MySpaceIM, SILC, Sametime, XMPP, Yahoo. But it doesn't allow you to connect to Facebook by default. You'll need to install a plugin for that. The plugin is here, instructions are pretty straight forward. Turn Pidgin off, install the plugin; it'll ask you if you want to launch Pidgin and you can be back on chat in less than a minute. Once you have installed the plugin, Go to the Account manager, and click to add an account. One of the options will now be Facebook.
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This is very handy for you to streamline your chats along with other networks you participate in, makes things simpler as things in general are becoming more complicated. In addition to chat, this also notifies you of status updates and you have Pidgin's built-in ability to save chats into the log, this is great if you're doing business over FB. Good luck.
As I mentioned earlier, if you're using a Mac you should try Adium, it does all the major accounts and Facebook.