"If you don't use it constantly and if you're unwilling to pay anything, then it's even more worthless..."
Looks like the same phenomenon I've encountered. Facebook must be assigning an 'advertising-power' (viewer-scope) per post according to account activity, likes and comments. I know they do this for individuals' posts to the feed, according to likes and comments, but I'm not sure if they do it according to activity, and what difference they make between pages and private accounts. They would have motivation because this dissuades/dis-incentivizes non-participation.
Teaching a free online course here might draw many users and more activity, and get everyone familiar with the setup. Such a prospect got me using reddit which I wouldn't otherwise have bothered with.
But it seems many are content to act as news-outlets on the FB and group feeds. A common feed might be a good idea here.