Fossilbook Share definition Flag Noun - the slang name commonly given to Facebook upon the discovery that over half of the users are now age 35+ Oh my god, I saw that picture of your new orthotic shoes on your Fossilbook profile! Fossilbook by Russ68 June 16, 2011 0 0 Get merch Get the Fossilbook mug. Buy Now — $32.95 See more merch