Facebook Blue

Below are the 5 blues that Facebook uses. The second, Facebook Blue, is the primary color. The other 4 blues are accent colors. White and Black are also part of the palette.

If you're more interested in fonts, you can find one similar to the Facebook font here.

 

Facebook Dark Blue

Hex: #0e1f56
RGB: 14, 31, 91
CMYK: 85, 66, 0, 64
 

Facebook Blue

Hex: #3b5998
RGB: 59, 89, 152
CMYK: 61, 41, 0, 40
 

Facebook Medium Blue

Hex: #6d84b4
RGB: 109, 132, 180
CMYK: 39, 27, 0, 29
 

Facebook Light Blue

Hex: #afbdd4
RGB: 175, 189, 212
CMYK: 17, 11, 0, 17
 

Facebook Light Blue

Hex: #d8dfea
RGB: 216, 232, 234
CMYK: 8, 5, 0, 8
 

Facebook White

Hex: #ffffff
RGB: 255, 255, 255
CMYK: 0, 0, 0, 0

If you're interested, Facebook has a site dedicated to their branding where you can download logs and screenshots and get information about the right way to use Facebook resources.