[Digital Jedi]

Originally Posted by Brandon Sheley

I'd say a forum icon would be similar the favicon of the forum itself, and tailored to the companies brand..
I don't see one icon being used across the web for every type of forum.

There's some exceptions to that. For instance, you might already be reading an article or a blog post on the authors website. So along with your social media icons linking users to your profiles there, you also have a link to your forum profile/recent threads/etc. on that same domain. Using the icon of a site you're already on might seem redundant or unclear. Again, the RSS icon serves a similar function.