Colorblind people have problems distinguishing certain colors. Because blue is a color that Mark Zuckerberg can distinguish well, he chose it for Facebook. See how does it feel to be color blind.
Mark Zuckerberg can't see red nor green the way they are. And because he knew so he had limited options to choose from when creating Facebook. Blue is the only major color, among the three major colors, that Mark Zuckerberg can see well.
As a colorblind person, Mark Zuckerberg had very limited options including yellow, grey and blue. Blue seemed to him like the right choice.
According to Mark Zuckerberg, blue is the richest color he can see, as he can see all shades of blue.
Blue is a calming and relaxing color that puts the brain in a relaxed state. The color blue is also sometimes referred to as 'Nirvana' for the brain. This could have been one reason why Zuckerberg felt comfortable choosing it. See why Facebook changed its logo.
Most of the colors distract people but blue acts like a transparent palette 65% of the time. This is could be one reason why Facebook decided to stick to the color.
Blue is a color that promotes and communicates trust. While this wasn't the reason Mark Zuckerberg chose it, it still could have been the reason Facebook decided to stick to it.