Without Installing any plug-in you can add facebook share button in your WordPress theme. First of all open header.php file which is located in your template directory. add below code before the </head> tag and save header.php file.
<script src="http://static.ak.fbcdn.net/connect.php/js/FB.Share" type="text/javascript"></script>
Then open  single.php file, find  function and add below link after the content function’s “?>” php tag.

<a name="fb_share" type="button_count" share_url="<?php the_permalink(); ?>"></a>

Or you can add above link any where you like in your post.

- Posted by Ashraful