I think it depends what's being posted. Facebook does it both ways - in a message thread it shows new stuff at the bottom, on the news feed it shows new stuff

I think it depends what's being posted. Facebook does it both ways - in a message thread it shows new stuff at the bottom, on the news feed it shows new stuff at the top. I think the key difference is where you have stuff that should be read in a logical order, it is easier to have it listed top to bottom (since the individual messages will be read top to bottom). Where you have stuff where you can pick up the thread anywhere (like a news feed) it makes sense to have the new stuff at the top where it is more visible.