How does the author know this is due to specific issues at Facebook and isn't just due to the job market being as competitive as it has recently become? Perhaps

How does the author know this is due to specific issues at Facebook and isn't just due to the job market being as competitive as it has recently become? Perhaps it was difficult get people at facebooks to leave, but now, with the market as tight as is it and with compensation everywhere soaring, perhaps it's reached a level where even Facebook is seeing attrition.