You need to replace the URL paths with:
<?php the_permalink(); ?>
Also consider replacing the title or description text for each button with:
<?php the_title(); ?>
Here”s an example:
<!-- Twitter -->
<a href="http://twitter.com/share?url=<?php the_permalink(); ?>&text=<?php the_title(); ?>" target="_blank"><img src="http://www.simplesharebuttons.com/images/somacro/twitter.png" alt="Twitter" /></a>
This code has been tested within single.php with a standard loop.