Dec 11, 2007An algo change has occurred with the SEO of Subdomains v Domains. Matt Cutts posted 10 December 2007 that subdomains were less likely to show up for a search where they were competing for the same search phrase as its main domain.
I especially noted this for the SEO of my forum.time2dine.co.nz. The forums page on Auckland Restaurants used to show on the search results for "Auckland Restaurants" - no longer, and this has only happened in the last few weeks.
My previous advice has been that for websites with many parts to them - forum, blog, shopping cart... that each should be on its own subdomain so they could rank in their own right for a given search phrase. Ebay has shown up for the top ten for many search phrases in the past, given its use of subdomains. Your shopping cart for a widget could rank as well as your blog about the widget.
However, this is obviously not a good policy now. It is likely that your domain will be the only one showing for the search result.
If you are blogging about widgets as well as having a shop about widgets, having the blog in a subdirectory could get the blog listed top for that widget phrase. That blog entry might not be as good at selling your widget as your shopping page is. It could be better that you therefore had the blog on a totally separate domain.
My advice about having separate subdomains for separate distinct categories still has merit (just). You need to make sure that the products/categories have no overlapping search phrases, otherwise you may end up having one subdomain not shown. Subdomains may be a risky policy. Since they have been abused in the past, any benefit from the main domain authority may be reduced in the future. At least this is what has happened in the latest Google algo change.
Remember - don't have sitewide links between any two domains. I use rel=nofollow on such links to keep websites apart from a Google perspective.
SEO for Multi Language sitesIt is possible to select different country preferences in Google webmaster tools for subdomains of generic (non country) domains. It is still my advice that using subdomains for different languages is beneficial. Don't use separate subdomains for say Australian and US versions. You should be able to safely use for totally different languages.
matt Cutts on use of subdomains for multi language
1. ccTLDS such as example.fr or example.de
2. After than, subdomains such as fr.example.com or de.example.com.
3. If that’s not possible, I’d use subdirectories such as example.com/fr/ or example.com/de/