Can someone explain to me why, when I log on to FB more than once within a few minutes, I'll see old and new posts all mixed up?

At the top of your news feed is a “SORT:” drop-down (“Top Stories” and “Most Recent”). If you sort by “Most Recent”, you should really see a list of posts sorted by post date/time descending. The algorithm they use for “Top Stories” is a mystery to me. Your guess is as good as mine.

FB is a law unto itself. I find it dense, cumbersome and very annoying.

I second what @janbb said, and I’ll add “it’s just like family.”

As Tom says you have to tell it to sort recent stories first. What I don’t understand is why I can’t use that as the default setting, why does it keep reverting to Top Stories instead. I think it is a conspiracy.

If you download a free add-on to Chrome and other browsers called SocialFixer it makes some of the annoying features of FB more bearable, e.g., you can keep the default at Recent Stories. There are other features that you can tweak as well.