To answer a question that arose in another forum for admins, take a look at the footer of the vBulletin.com page:
It seems appropriate that amid the Google+, Facebook, Twitter, and YouTube icons that there be an icon that provides a visual clue and a clickable shortcut to a forum associated with the brand.
But those icons tell you where you are going- Facebook, Twitter, Google+, etc... they are all external sites.
A generic forum icon- I don't know where that is taking me? Is it to an off-site forum? A forum on the site I'm currently browsing? Any forum of decent size probably has its own logo that they'd use for a custom icon already- between favicon and apple touch images they probably have something they could adopt.
So this isn't really for external links, the only similar icon out there really is the RSS icon, possibly a generic blog icon- you want a generic forum icon. This isn't going to compete with Google and Twitter, only RSS and Blogs.