Two hundred years ago, we processed food to survive the winter, now we do it to make a few extra bucks from each ton of corn and wheat.
I am optimistic that the best social media sites have yet to be designed--rather than optimizing to squeeze the most engagement, ad dollars, or whatever, I like to imagine algorithms that seek to encourage pro-social behavior, friendliness, happiness, fun, healthy habits. We may discover that Twitter and Facebook are fundamentally incapable of doing these things as well as their future replacement, whatever that may be.
I don't have an exact answer to that, but what I do know for sure is that the good has been on a downward trajectory, and the bad on an even steeper upward one, for years. Even if they haven't actually crossed yet, it seems only a matter of time before they will.
The direction the net is currently going looks a bit worrying, I definitely agree. But I would not want to miss all the awesome advantages it brings.