This problem can be tricky. Discourse (the software that runs these forums) uses a featured called “OneBox” that, in turn, utilizes Facebook’s OpenGraph to embed web page previews.
I used one of your URLs to test in their debugger tool, and it says that your site is missing the required Facebook App ID:
To my knowledge, you don’t strictly need to include that attribute (my site doesn’t include that, yet my embeds work just fine). That’s because I’m using a plugin (MashShare) that includes the OpenGraph meta tags into my site.
You may want to look into a plugin like MashShare or another one that specifically adds in Open Graph meta tags into your site, like this: