One out of six of them.
While it seems impossible that one can go from treating someone as the most important thing in your life to not speaking within a matter of weeks, days or hours, sometimes it makes sense. The promise to stay friends is difficult, especially if you did not know the other person before you were dating. Afterwards, there is no hole for them to go back into as they weren't in your life aside from as your "other half". As such, they leave.
Sad, but true.