A Facebook poke is a free feature that allows you to send a virtual "Hey, I was here" message. The poke shows up on your friend's page as an icon that denotes who poked him and offers the options "Remove Poke" or "Poke Back." If you receive the icon on your page, only you -- not the visitors to your page -- will see it, and if you don't poke back or remove it, the person who sent it can't poke you again. Once you hide the poke or poke back, however, that person is free to poke you again. Facebook's settings allow you to request an email when you receive a poke.
When you visit a friend's Facebook homepage, look in the upper high-hand corner for the gray button that says "Poke" and contains an image of a finger poking. The button is directly to the right of the "Send Message" button. Click "Poke"; no further action is required. When you visit your own homepage, you won't see the poke button because you can't poke yourself; on your page, you'll find the "Edit Profile" button instead.
You are allowed to poke someone who is a confirmed friend of yours, a friend of a confirmed friend or a person in a shared network. The same three categories of Facebook users can poke you as well.
Several free programs allow you to automatically poke friends. Facebook Autopoke 3.5 pokes back friends listed on your home page. SuperPoke lets you poke with "little extra" icons such as kiss, high five, hug, cuddle or spank, among others. To send a poke from your android phone, try AutoPoke, available from AndroLib; the program requires that you have SMS alerts enabled on your Facebook account.