You could face a similar problem I had recently running WordPress using Nginx + php-fpm. When a share button (not the original ones, but the ones made by Jetpack) is clicked, a smaller new window is opened with the post in it, but nothing is shared.
The solution is very easy, you have to replace the following in your Nginx config file.
try_files $uri $uri/ /index.php;
try_files $uri $uri/ /index.php?$args;
More information about using WordPress with Nginx at http://codex.wordpress.org/Nginx#General_WordPress_rules