If you insist on chatting with Facebook and Gmail buddies outside a browser window, then it's worth taking Gabtastik for a spin, though as far as instant messaging applications go, the most interesting thing about Gabtastik is the technology.
As a Site Specific Browser (SSB) app based on Mozilla's Prism project, Gabtastik, which released a Windows beta this week, tethers the Web chat ecosystem to a desktop application. From this standpoint, Gabtastik is a success. The Facebook chat and Google Talk GUIs are spot-on. The essential problem is that the chatting isn't very interesting, and the program doesn't do much beyond keeping each site's proprietary chat actions and experience intact as is its nature as an specific, no-frills mini browser app. But good luck multichatting with contacts from both Facebook and Google--you'll have to switch between the two interfaces each time, an either/or prospectus that doesn't lend itself well to an unobstructed workflow and is worsened by the weak message notification system.
If Gabtastik can manage to create a unified interface with tabbed chatting, intuitive notification, and program preferences, and then integrate that with Meebo, Yahoo Messenger online, and Trillian Astra--whenever that comes out--it'll at least have a shot at gaining traction as a niche multinetwork Web chat client.
Gastastik for Mac has also been reviewed.