Remember, facebook is about one thing, making money for Zuckerberg and now the shareholders.
AT ANY COST. Since Facebook is not a for-pay site like Match.com where you pay for basic functionality, nor an ecommerce site like amazon where it can take a cut of sales, it can only make money in one of two ways... displaying advertising to its userbase, or collecting information about its userbase to sell to advertisers.
It does both, of course. That, in and of itself, is not the problem; Google, Yahoo, Amazon, all also do the same thing. The problem is that they also have other revenue streams, Google has its search and ad businesses inhouse, Amazon has sales, and Yahoo has an almost insurmountable news delivery platform. Facebook has nothing but its social media content and a few patents. so it HAS to bang those two revenue sources as hard and often as it can.
Therefore it has to keep people on the site for as long as possible, and it has to keep them moving around the site and building up profile data as much as possible. How can it do this? particularly when it does not generate any content of its own like yahoo, nor sell things like amazon, nor provide info on the outside world like google? It does so by using
Skinner Box techniques. The like button is a fast reward system pioneered in Skinner's research, purpose designed to encourage short, quippy, memetic, reshareable content, because a lot of likes is equivalent to social acceptance, and social acceptance is a reward in itself. Phrases like #yolo and icebucket challenge and nipslip were all but unheard ten years ago. In that time, long form communications still ruled. Chat rooms and message boards reigned supreme.
Then Zuckerberg came along. overnight Facebook and small-factor clones like tumblr and reditt began to put a premium on skinner-box techniques. Why? Why encourage this meme-heavy behavior? It's highly profitable... you move people around your service quickly searching for more likes to see and more things to get liked by others, and with each click on your site, or on another site tied into your system (facebook sign-in, likes on third party sites, digs, redits, upvotes, etc) you gain more saleable information about people doing the clicking. You also show them a barrage of advertising, particularly in highly trafficked areas, and each click of those ads, or even each non-click view, generates ad revenue. All of this is done without you generating one whit of actual content beyond the platform itself.
In this carefully structured environment, certain types of users, archetypes, are key. You must have
content generators... CNN, National Geographic, Microsoft, etc... companies who are willing to post their material to Facebook, also for free. You must also have
engagers; people who say to the rest of the world "come and look at this thing I have found and liked." And you must also have
conversation drivers who will keep people coming back to the discussion and thus keep down the need for the first two types of people who are more difficult to procure and retain to the site. Unfortunately for us normal, level-headed types, the most effective conversation drivers are trolls like Randal, or SlickNickShady, or dozens of others. They have
sociopathic personalities that generate joy from the suffering and discomfort of others, and psychological tools that make them very accomplished at eliciting those responses. A troll can whip up, in just two or three posts, an internet argument (just like this one here) which will roll on for days without any further input, a roiling conversation with little actual merit or point, because a calm and reasoned discussion usually ends within a few posts as people go "yeah, guess you were right."
So... if a conversation driver is key to maintaining site activity (is profitable), and a troll is a conversation driver.... then a troll, ipso facto, is profitable to facebook and other companies of its ilk. And then encouraging them and their behaviors is also profitable. So it is quite literally in Facebook's best interest to encourage behavior You and I see as insane (clinical sociopathy exercised by internet trolls) in order to turn a profit. Because insanity drives traffic, and traffic drives profits, and encouraging more people to fall into the insane skinner box trap is encouraging more profit.