A poke on Facebook is the equivalent of tapping someone on the shoulder to say hi. It is a way to let other people know that you are thinking of them without going through all the trouble of sending private messages or posting publicly to their online wall.
As of 2014, to poke someone, a Facebook user must select him and go to his homepage. Once on the person's homepage, the user finds the Poke option in the drop-down menu under the button to the right of the Message button. The Poke option is a quirky, fun method that allows people to stay in touch with each other on the social networking site.