Facebook Poke, one of the oldest and mysterious feature in the social network, has got an add-on which lets you see how many time you’ve poked someone.
You can see your pokes activity on your pokes page. It will begin showing you how many times you’ve poked back and forth with a friend after you’ve poked them more than once. The poke counts will be shown next to “[name] poked you” labeling “[number] times in a row!”.

Top image:Cheezburger
I think ‘poke’ is used when you wanted to chat with someone and you see him/her online but you’re not sure if he/she wants to chat with you. So you’ll poke that someone then wait if he/she will poke back for a chat. :)
For people that have been around since early Facebook we remember what the original function of the poke was. Back when Facebook was just for university students your profile could be viewed by anyone in your network but not by anyone outside of it. It was supposed to be a virtual yearbook. But if you wanted to let someone from outside your network know you were interested in being friends you would poke them. This let them view your full profile without being your friend or being a part of your network. You still couldn’t see their profile though unless they poked you back or added you as a friend.
Now days with open networks and the ability to make your profile private to everyone the poke has lost its function and is just something to do for fun.