Varnish and Pageview Add-On
The Mashshare Pageview add-on is using a custom ajax url which is faster than regular the /wp-admin/ajax.php
The path to it is
id parameter's value is the ID of the post which changes depending on the viewed post.
If you are using any web accelerator like Varnish make that you exclude this path from caching.
Don't forget to make sure that Varnish is purging its cache after an amount of time (a few hours), otherwise the page views will not be counted.