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Facebook Chat could well be the world's largest chat protocol, as the number of available users surpass that of ICQ and Windows Live Messenger. And considering it is so widely available as an open protocol, anyone with a smartphone can download an application to contact their friends on the go.
So if you jacked in other instant messengers to keep all of your social activity all in one place, then you will be missing out. Then again, so many people 'turn off' Facebook chat anyway, so they cannot be bothered, one might question whether it is as popular as one might think.