Color blindness, otherwise known as color vision deficiency, affects about 4.5% of the entire human population. About 1 in 12 men and 1 in 200 women suffer from it worldwide. Mark Zuckerberg, CEO of the world's largest social network, Facebook, falls under this statistic.
It's actually the reason why Facebook is its famous brand of blue, because that's the shade of color that he sees best. It's actually not a bad choice, because in the design and marketing world, blue is seen as a powerful, trustworthy color - quite suitable for a website that connects more than one billion people at the time of writing.
The New Yorker wrote in their 20th September, 2010 issue about how Zuckerberg discovered his red-green color-blindness:
[Mark Zuckerberg] walked into the house, which is painted in various shades of blue and beige, except for the kitchen, which is a vibrant yellow. 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." Standing in his kitchen, leaning over the sink, he offered me a glass of water.