Here's how to detect whether a Facebook ads placement has become a problem.
Usually this happens when your results are too good to be true. Use Breakdown by Placements to get a better sense of performance by placement.
Look at the distribution of actions and budget by placement, but also look at the quality of those actions. Are you getting what you're wanting? Is Facebook spending all of your money on one placement?
What you don't want to do is micro-manage placements. Don't go in and pick out the two or three highest-performing placements and turn off everything else.
Why? The placements where you're spending very little often help keep your costs down. If you force Facebook to focus only on News Feed, for example, expect your costs to go up.