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.
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 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.
If someone pokes a user, it opens up their Facebook profile to be seen by that person for 4 days. The recipient of the poke will receive a notification allowing them to see the person who poked them. No one else will know if one of their friends has been poked.
A person cannot be poked again by the same person unless the poke is removed, or if the user pokes back the friend who poked him first.