I was just wondering if there are any benefits of having a forum under a subdomain. I run a forum, but it is simply under a directory. I want to add other things to the site, such as a social network and music management system, and I am wondering if my structure should be changed. I use vbulletin for the forum under a /forum directory, but I am going to have a social network and music management system built for me, and it will be integrated with vbulletin. If I was to put the social network in a subdomain, would integration be possible if vbulletin is not on the same subdomain? And, does it even matter if I use subdomains or not? I am just wondering why some big sites, such as myspace, choose to use subdomains for their message boards.
I agree with the subfolders option. A sub-domain is considered a new site from the search engines perspective. Plus I think having your existing content "grow" with user comments makes it much more valuable. Consider embedding your forum functionality into your existing pages.