I would have thought a lower frequency of posting would be an indication of a greater weighting of the content when something was posted. Of course, as far as keeping eyeballs for a long time, that's not what keeps people on the feed. Maybe because those kind of posts are more likely to get people out of the feed and onto direct messaging instead? Rather than lots of stuff (reposts and shares) to scroll fast by and thus more space for ads to also scroll by.