Here's what the New York Times says:
Particularly intriguing is that when the algorithm fails, it looks as if the relationship is in trouble. A couple in a declared relationship and without a high dispersion on the site are 50 percent more likely to break up over the next two months than a couple with a high dispersion, the researchers found. (Their research tracked the users every two months for two years.)
Simple enough. Too bad Mark Zuckerberg doesn't make a push notification for when you need to cut it off.