Meebo.com An Awesome chat web based chat client which allows you to chat on your Facebook, Yahoo!, My Space, Google Talk, Windows Live, AIM, ICQ, Jabber, Flixster, My Yearbook, Zinch, ibeatyou, and many more, just get Registerthere and Start chatting.
Meebo also available in Android, Apple, Blackberry Apps here and for nokia and other phone can go to this link.

if you feel inconvenient then we have many way which let's you start Facebook chat conversation from anywhere without even opening facebook.com in browser.

Gabtasktik it is a minimal desktop chat client for facebook which looks exactly web version of facebook chat version on web browser, you also send you notification and chat request  via system tray.

Digsby It's damm Awesome all in one usefull chat client which helps you to update whatever happening on your facebook, Gtalk, Twitter, linkedln, by provide notification on system tray.
   It is good if you have multiple account on any social networking site like facebook, it will let you help to associate all of your friends and display in one windows. Facebook doesn't allow audio or video chat but digsby bring back this feature indirectly via tokbox plug-in .
Digsby available for Mac OS X, Windows and linux. get it here.

Pidgin It is one of my favorite universal chat like and i mostly use it on linux OS. It is same like Meebo but unlike it's desktop client for windows & linux, it also let you associate all of your multiple account of various social networking website like facebook, Gtalk, MSN, AIM, ICQ, IRC, Yahoo!, My Space, and display on one screen . 

Adium an Awesome desktop chat client for Mac OS X users, adium is best choice for mac users to chat on facebook with friends from desktop. Just like Pidgin and digsby Adium also support many IM network like Facebook, Gtalk, Aol, AIM, ICQ, Yahoo!, Skype, twitter, and Jabber (XMPP) including Windows Live Messenger.