Josh Constine writes:

If every friend and Page were treated equally, it’s natural and in fact unavoidable that organic reach — the percentage of their friends or fans that see their feed posts — will decrease over time. And it is. A study from News Feed optimization service EdgeRank Checker of 50,000 posts by 1,000 Pages shows organic reach per fan (median) has steadily declined:

• Feb 2012 = 16%
• Sep 2013 = 12.60%
• Nov 2013 = 10.15%
• Dec 2013 = 7.83%
• Mar 2014 = 6.51%

The roughly 50% decline in reach over the past year matches the 50% increase in Page Likes per typical Facebook user over the same time period. As people Like more Pages, the organic reach of each drops.

But all Pages and people are not treated equally because Facebook’s goal is to show people the most engaging posts out of all the ones they could see each day. Facebook’s ability to earn money showing ads and pursue its mission to connect the world hinges on people coming back because they see interesting content there and don’t get bored.

I really hate when they put an older post back to the top of the feed, even when in "Most Recent" mode.  I've already seen it, and don't want to see it again, nor do I have time to, it's a disrespect of my time.

I guess they do that because of new comments on the post?

More often there is no new comments, maybe a new "like"?  If I really want to keep abreast of comments on a post, I will comment on it, and then I will get email for every future comment.

True. You're right, there's no good explanation for it.

In my feed this am:

• 59,771 people like this.

• 175,890 shares

• 2660 comments

That's awesome. And compared to the 1.2 billion monthly users, that population is practically zero.

That's a lot of "Whales" sharing ;)