For the social feed there are two options for the order that posts are displayed in Latest Post and Recent Activity.
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The option can be changed using the Sort by option at the top of the social feed.

The latest post sort option displays posts in the order that they were first created, with the newest at the top.

The recent activity sort option also takes account of when comments on posts were created, so a post that has been commented on since another was created will now appear before it on the social feed.