It's pretty simple. I have added the addthis code to my forum pages. When you click the facebook share button(or any other for that matter), the popup box cannot display the information in the post/page.
example:
If you leave the # character in the url, then try and share to Facebook, the comment box displays
I also have a problem with the share buttons not creating the proper popup box instead opening a whole new page/tab in the browser. The whole thing just doesn't work properly. Won't graba video that may be in a post, won't grab an image. It just sucks.
I have noticed one more thing as well. Kinda weird...
The original URL for the post looks like this:
http://towtalk.net/forum/#topic/To-stack-or-not-to-stack-.htm
If you permalink it once, it then looks like this:
http://towtalk.net/forum/topic/To-stack-or-not-to-stack-.htm#.URc_d2eFlt0
Then click it again, it looks like this:
http://towtalk.net/forum/topic/To-stack-or-not-to-stack-.htm
Now it seems to me in the social media times that we live in that one of the first considerations of building a forum script is to make sure it's compatible at least on a basic level with other social media engines so sharing, something we all do, can easily be integrated and used.
I have tried this before and have always had the same result. Trying to share a forum post is abysmal at best and if you don't 'clean up' the URL to start with, it fails miserably. Forums for most sites are where the bulk of the activity lies and it's that activity that can generate reciprocal usage and greatly increase your entire SEO face to the splinternet..