Facebook over the years has changed its logo multiple times it has stuck to the color that is still in use today. It is not a common practice to use this kind in web design but Facebook has got away with it very well and it appears that this color is here to stay.
PANTONE: PMS 7684 C
Hex Color: #3c5a99;
Facebook color has verified by facebookbrand.com where you can find more information about its other brands such as Instagram, WhatsApp, and Messenger.