I have an FB account because someone on another forum (of whom I am quite fond) convinced me to join. But I hardly use Facebook at all simply because I don't know what to say on it. And like you say, roygerdodger, I find I don't have a lot of exciting stuff to share in Facebook messages. Online, I personally prefer message boards to networking sites... those give me more time to think of a good reply, everyone isn't always expecting me to respond immediately, and there's more room for replies of substance.
I just experience Facebook as being too casual and transitory for my tastes.
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clarity of thought before rashness of action