Super Share wordpress social plugin by MasterBlogster shows the social sharing buttons in a popup box only when reader reaches the end of the article. It also contains a Feedburner subscription form.
Here is the
live demo of Master Blogster Super Share plugin
Plugin homepage:
http://masterblogster.com/plugins/super-share-wordpress-plugin
[It is highly recommended to update the plugin to the latest version i.e. 1.1]
Extract the zip file and just drop the contents in the wp-content/plugins/ directory of your WordPress installation and then activate the Plugin from Plugins page.