1 answer
@Kate : Chatter groups can be archived, but unfortunately not Chatter posts. Here it is in Idea stage. Please upvote it.
Chatter Post Automatic Archiving-Idea (https://success.salesforce.com/ideaView?id=08730000000DkmIAAS)
Chatter Group Archiving (https://help.salesforce.com/HTViewHelpDoc?id=collab_group_archive_admin.htm)
