Everything in the Facebook like mobile page, pop up windows, etc., are so blue because the founder of Facebook Zuckerberg was actually color blind, for him color blue is visible. It actually mentioned in New Yorker
Colors don’t matter much to Zuckerberg; a few years ago, he took an online test and realized that he was red-green color-blind. Blue is Facebook’s dominant color, because, as he said, “blue is the richest color for me – I can see all of blue.”
Emil Kostov says why Facebook maybe sticking with blue:
It is part of their well planned Marketing plan. Most colors distract and make the human eye focus on them while blue act as a transparent palette for 65% of time spending watching the main background aka index space. It is well known also that blue color is the nirvana for the brain, as well green. Well, their choice make the competition envy most of the time.