A facebook "Poke" is a meaningless feature of Facebook which many Facebook users still find confusing. I have some facebook friends who just can't stop poking, poking me. Guess what I used to do? I keep poking and poking them back till they give up...lol. While some people see poking as annoying, some just see it as fun.
Poke most commonly refers to the action of tapping and/or softly jabbing an object or person using a finger, stick, or similar object etc
The "Facebook Poke" according to my own personal understanding, is a feature of Facebook whose sole purpose is to attract the attention of another user. As pokes attract the attention of a user, they can be used for various different things and thus have different meanings. You can, for instance, poke a friend just as a way of saying “Hello. I am still alive”.
The only thing that happens when you poke someone is that this person receives a poke alert on his or her home page. Users can only poke a confirmed friend, someone in a shared network, or a friend of a friend.
The poke alert you receive when you are poked is just a poke icon with a message that says, “You were pocked by,” and the name and picture of the poker. It also has an option to “Poke Back.” Using this option, sometimes people have “poke wars,” where they keep virtually poking and poking each other until someone gives up.
Might sound funny, but that’s what people do. I do that too...lol
Source: Yahoo Answers