Facebook hires some of the top software developers and engineers on the planet, if not leaking plaintext passwords is too high of an expectation for them then nothing that isn't public knowledge should ever be put into any computer system. As a profession we should demand our peers do better than this.
We're all people of varying skills, I would never assume an innate high bar for any activity a human does - only by requiring the bar be maintained at a level and regularly checking and enforcing that requirement can we be reasonably sure it is - and this isn't just `echo $password` the way these passwords got into the log file is (from what I've been able to discern) pretty obscure and round about - Facebook is absolutely responsible and needs to be held to account, but the mistake is understandable.
