Haha, I think our bandwidth usage would be a BIT higher than that, since we're exchanging XMPP messages a lot more often than your standard non-graphical chat, just for position updates alone. It's fairly well optimized, so at least you don't transmit anything if you're standing still (or even running in a straight line, motion prediction ftw), but pretty chatty if you're jumping around and emoting a lot.
"Changes presence 3 times an hour" is probably a little low. We do presence changes whenever you zone as well as if you switch costumes. That estimate probably also doesn't assume being in 3 MUC rooms with 50 other people each.