First of all I would like to thank you for this great plugin! I installed it into WordPress and it worked exactly as advertised.
However, the plugin seems to recalculate the number of comments on every reload. Especially for the main page with a lot of posts that means lots of Facebook requests. The plugin led to a noteable slowdown which was not tolerable, so I had to deactivate it. My suggestion is to implement some kind of caching mechanism, so that a check is only performed in certain intervals.
With an addition like that the plugin would be perfect!