Chat should go on the bottom. Why? People (at least people reading English) read top-to-bottom, and chat messages very rarely make sense out of context: when so

Chat should go on the bottom. Why? People (at least people reading English) read top-to-bottom, and chat messages very rarely make sense out of context: when someone is reading a chat message, they're almost certainly reading a conversation, not a single chat message. The other examples you mentioned, like Facebook/Twitter posts, are different because they're usually standalone and don't have to be read together with other posts. (You'll notice that new comments on Facebook posts appear at the bottom, because those are meant to be read together and in order.) The one example you mentioned that doesn't really fit here is email threads, which are displayed with new messages at the bottom in interfaces like Gmail.