Algorithmic feed + moderators. This is what makes FB, twitter such horrible places for building a real community. Each thread in a FB group is valued but it's ability to create more traffic and more ads and not because of it's true value to the community. The result is that what comes in your feed is usually the feeds that generate extreme reactions from different sides. On forums if it's up you know it's a real discussion and people have add true value to it otherwise the moderators would have intervened. If something was asked and answered it saves as a reference for next time. In social media there is no such thing. The algorithms wants you to recreate the same threads again and again just to make sure more traffic will be generated. It's no coincidence that search is not useful in FB groups.