Some time ago, it was a concern that using a subdomain or a folder would bring the same juice to your main domain. E.g.: I want to advertise / optimize www.domain.com Using something.domain.com was the same as www.domain.com/something But according to the new google algorythm (caffeine), does this remain? I wish to create a wiki for our users to contribute. A wiki always bring much content. Shall I use wiki.domain.com , or www.domain.com/wiki ?
General advice: If the content is related to your website go with an inner folder within your site instead of a subdomain.. otherwise go with a separate subdomain for each different kind of content
I think that subdomain is better, for wikis, forums, blogs because they are different from the main site.
Subdomain - Google will look at as a new separated site and has nothing to do with the main domain. However, Google has devalued the subdomain value already, before their latest "Caffeine" update. Even looking at a new site, subdomain need to work much harder to gain authorize status, plus the main domain weight has been completely cut-off from being calcaulate the value of subdomain like before. Subfolder - Google would see this as a part of main domain.
I would go with wiki as a subdomain. This way you have a separate mail server, stat server, dbase server and so on. It's something you can build a community around. From an SEO point of view, one approach is probably not much different from the other except that some directories that don't allow deep links will allow the subdomain.
From the SEO point of view, google treats subdomain as a seperate domain... so if you don't mind promoting (getting links) to the subdomain then go with it but if not then go with subfolder.