This is a very nice to interact and allow people to share your post. I love this plugin where readers like your post and published on their wall. It consist few lines of code and easy steps to follow. Follow the steps and you can install it even without the basic html knowledge.
Two easy steps to follow.
1. Insert this code inside the head tag where is the content dlysen is your facebook user name.
<meta property="fb:admins" content="dlysen"/>
2. Insert this code on the body tag
<div id="fb-root"> </div> <script src="http://connect.facebook.net/en_US/all.js#appId=125026257515591&xfbml=1"> </script> <fb:comments href="{ShortURL}" num_posts="5" width="470"> </fb:comments>

Facebook Social Plugin Preview
Easy guide to install in tumblr theme
Customize your tumblr http://www.tumblr.com/customize and navigate on the theme.
Insert this code between html head tag. The first line create an option on the appearance setting of tumblr if you want to show the plugin or not.
The second line tells the script who is the admin to moderate the comments. Remember the content attribute on the second line is your Facebook user id or user name.
I recommend to retype the code.
<meta name="text:Facebook Username" content="dlysen"/>
<meta property="fb:admins" content="dlysen"/>
Then insert the code below nicely in your content. It appears on the permalink page. I will explain the details here.
The first line is just a code comment, the second line open the block where it tells tumblr to load it only on the permalink page.
The third line block enable the social plugin when you add your Facebook user name on Appearance Setting.
The fourth line is the primary code we need to display the plugin.
The fifth and the last code is just a closing block of the previous block.
<!-- Facebook Social Plugin -->
{block:PermalinkPage}
{block:IfFacebookUsername}
<div id="fb-root"> </div>
<script src="http://connect.facebook.net/en_US/all.js#appId=125026257515591&xfbml=1"> </script>
<fb:comments href="{ShortURL}" num_posts="5" width="470"> </fb:comments>
{/block:IfFacebookUsername}
{/block:PermalinkPage}
Here is the code for the facebook like box on tumblr. you can put this in between {block:Posts} [ fb code here ] and . . . {/block:Posts}
<div id="fb-root"> </div> <script src="http://connect.facebook.net/en_US/all.js#appId=167545586638609&xfbml=1"> </script> <fb:like href="{Permalink}" send="true" layout="standard" width="421" show_faces="true" font="arial"> </fb:like>
Navigate on the appearance setting and add facebook user name. Don’t forget to save.
You can find all how to information on http://developers.facebook.com/docs/plugins/
You can preview how it works on my tumblr blog Make A Difference Tumblr and you can download Tumblr Ads Theme text to get the html code.