So for example Facebook pages like "Humans of New York" or "I Fucking Love Science" or "Amy Poehler's Smart Girls" still get popped into a lot of timelines despite (to my knowledge) not paying at all for boosting. Why? Because a lot of people love that content.
But if some other Facebook page saw a huge drop-off that stayed dropped off, then maybe that content is just not as interesting as other pages (like the ones above for example).
And in fact this is why Facebook made changes to page content visibility in the first place. Most pages posted boring obvious ads instead of interesting content.
Pages can still pay for higher visibility, but those are ads. Just like people can pay to "get into the New York Times"--via ads, not articles. And just like the NY Times, there is only a fixed amount of ad inventory at Facebook. Even when pages are paying, Facebook limits their audience, in order to keep the audience happy. No one wants to see a newsfeed full of 100% ads. Or even 50% ads. Or 25%.
Just go and count how many things you've liked so far. I have 821 "items" liked, most of them fanpages. If every one of them would be able to reach me with all their posts, I'd quit Facebook on the spot. The users are not there for the fanpages. That's why Facebook won't let companies hit everyone. Now whether or not they should allow people to pay for wider reach, that's questionable, but then again - it at least makes people prioritize. If you have a limited marketing budget, you will tend to pay for and thus prioritize more important communication.
But then again, Facebook has to get the money for their servers from somewhere (or at least enough of it to attract more investors). They could've chosen worse monetization strategies.
No, it wasnt like that when most of the Pages acquired the bulk of their followers, they could reach all their audience freely. It was a shitty idea solely to make money, it didn't "add value" to users. There were other, more meaningful ways to implement this, e.g. show Page posts on a separate tab, have limits of posts/day etc. Paying for views is a bad strategy to improve quality; imagine if HN worked like that.