This partly relates to the history of these media. Chatrooms date from the terminal days when everything scrolled from the bottom (because terminal behaviour was based on that of line printers). So there is an expectation for them to behave this way.
I’d dispute that forums put latest messages at the top. At the “latest threads” page the most recent thread activity is usually first (after any pinned threads) but within the thread page itself, the latest message is usually last by default, although this depends on the forum.
In general it is better to have messages in date order in any medium because it makes more sense when reading from the start.
Chat rooms tend to be fast and interactive, so require the user’s attention at all times, so the user is watching for new messages and ready to type their reply. Forums, blogs and social media tend to be updated and visited less often, every few hours for example, so it makes it easier for users to dip in and out if the new content is at the top.
Also with social media the posts may not be related to each other, e.g. a FB wall status is an island of content with its comments attached (often in a semi chronological order) so it makes sense to show the most recent first, so visitors see new content. Twitter on the other hand has some conversational aspects and they have addressed this with their “tweets and replies” view but the emphasis is still on new content.
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