I thought that maybe I could backdate posts with Friendica, but that's not possible.

What I'm doing for now is posting things with "Archive Post" along with the title and posting date in the title field. I may integrate discussion under posts from Facebook or a thread on Twitter into the post itself if it adds to the original post, clean up typos and such, but will try to leave the substance the same.

"Archive Post: Something of other (2001-01-01)"

The categories will include "Archive Post" and be set to non-public just to prevent distribution of the old posts. Nobody really cares about these than me.

Copy the post over, archive it on Facebook.

I have a Twitter archive from last year, before I killed that off. I might go through that a bit, but the motivation there is low.