I would guess that people are interested in the newest context available. In something like Twitter or Identica, messages are largely unrelated to each other,

I would guess that people are interested in the newest context available. In something like Twitter or Identica, messages are largely unrelated to each other, therefore the unit of individual context is a message, and the newest messages appear at the top. In something like IRC, messages tend to be very heavily interrelated, and the unit of individual context is the entire channel - or at least the past five or ten minutes. Until IRC clients learn to split channel contents by semantic context, they present the entire context (i.e. channel history) in chronological order for your perusal. In something like email, messages tend to be somewhat interrelated, but because each message has links to its related messages, it's easy to break up a mailbox into threads. Generally, I would expect this kind of system to show the newest thread at the top, but the individual messages in that thread chronologically. Since some email threads grow to enormous lengths, it's not always useful to have the first two thirds of the page occupied by messages posted a month ago, when all the activity is occurring in the leaves. In this situation, it might be useful to only construct threads based on activity in the past week or so - that way, the individual sub-threads of the Monster Thread can take their appropriate places among any other threads that have occurred recently.