To "like" something on facebook. Telling someone that you like something doesn't necessairly mean that you liked it on the book, but saying "I thumbed that" doesn't need clarification.
damn you people who dont know what a thumb is...a thumb is a FINGER, it is also a DIGIT as well as all of your other fingers/digits, they are all the same...i find it sad that all of you losers dont know what a damn thumb is...wow!