The Facebook logo was designed by Joe Kral and Cuban Coucil. It is a modification of Eric Olson’s Klavika, with some altered lettershapes for a tighter fit and a taming of Klavika’s very distinctive ‘k’. In the same way that Facebook represented a major shift in culture and technology in the early 2000s, Klavika did the same for typeface design — well crafted but blatantly simple, monolinear, square, nearly devoid of any calligraphic ancestry.
Comparison by Stephen Coles. This is an old image, sorry. Will try to make it bigger soon. License: CC BY.
Klavika (above) and the Facebook logo (below) showing adjustments to the ‘f’ arch, the ‘fa’ connection, the ‘k’, and widths of other letters.