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While I can't comment on how SEOmoz did it, here is a link on how to create a similar app using the Drupal framework: http://blog.attomsoft.com/drupal/122/how-to-build-a-stackoverflow-or-yahoo-answers-like-system-using-drupal-part1

I agree with the previous responses that the blog should probably be a subsection. A key question to ask is whether the blog will attract links either naturally or through a link building program.  A blog that stands a change of obtaining significant traffic and links, and features content that is independent from the main website is a candidate for a root level domain. However, from your brief descriiption, this does not seem to be the case with your blog

I have about 0.02%-0.06% depending on the client for mail.google.com but it reallly depends on your keywords you are targeting aswell. On the content network don't bother looking at ctr unless you want to test a new ad

Keywords in the domain name are not carrying as much value as they once did.  (See the Matt Cutts video below). The value you that you would get from having your keyword in the domain of a microniche is pretty small. I think, if you had these two options: bodybuildingfortheelderly.com MyMainSite/bodybuildingfortheelderly ...you could still rank the second one really well.  And, any links you build to the second one would help the entire domain. watch?v=rAWFv43qubI