Color is important to Brand. If you think Facebook has chosen Blue for its theme just like that then you’re wrong. The Facebook has a very genuine and valid reason for having Blue skin.
According to The New Yorker, the reason is simple. It’s because Mark Zuckerberg is red-green colorblind. This means that blue is the color Mark can see the best. In his own words Zuck says: “Blue is the richest color for me; I can see all of blue.”