Most of the people know what is facebook so there is no need of describing it.So do you want the facebook share button?If yes then go to http://www.facebook.com/facebook-widgets/share.php and choose the style you want.You will get the code of the button from there but they have not told how to apply it.For that follow the steps given below:-
1.Go to Layout—> Edit Html—> Click on Expand Widget Templates check box.
2.Find the following code:-
<div class='post-header-line-1'/>
3.And after it, paste your button code you got from the site.
4.Did you got it?If no then find this:-
<data:post.body/>
5.And then paste the button code before it.
