Are you sure you haven't got that around the wrong way? If I check Facebook right now, I will have missed everything that happened in between now and the last time .. unless I scroll and scroll and scroll.

But if I check email right now, I can come back and see the same emails that I haven't read yet, a week later, sitting in the same approximate location.

Also, umm .. really, this: "Unlike e-mail, people can join the conversation without an explicit invitation."

.. just doesn't seem quite right. Ever been on a proper listserv?

Actually, your observation - and don't take it wrong, but I think you got it .. not .. quite .. right - is fascinating. You actually have it completely inverted, in my opinion - is this because you learned to use Facebook before you learned to (properly) use email? Not trying to be offensive, but this inversion seems pretty obtuse ..