We operate a 12-year-old group of branded Web sites that provide subject-specific technology tips and techniques. One is for mobile computers, one is for email, one is for Lotus IT professionals, etc.
Right now, they each have their own unique domain name, and are linked together by a "related sites" ribbon at the top of each page. Here's how they're ranked right now:
topic1.com: Page rank 5/10, Alexa U.S. rank 399,760
topic2.com: Page rank 5/10, Alexa U.S. rank 176,373
topic3.com: Page rank 4/10, Alexa U.S. rank 432,522
topic4.com: Page rank 4/10, Alexa U.S. rank 632,112
topic5.com: Page rank 4/10, Alexa U.S. rank 180,184
These all run in a legacy CMS and have about 100,000 pages combined. My big project is moving all of these to WordPress. In doing so, I'm also considering some SEO issues, one of the biggest being whether it's better for SEO (and, hence, traffic) to combine all five sites into one super-site, with each topic a subdirectory. So instead of topic1.com, topic2.com, etc., we'd have:
Obviously, this will require some quality mod_rewrite action, but the big question is this: is it better to combine them all into one site, or worse? Will I lose SEO equity by combining them, or gain SEO equity because they all have incoming links with very little cross-over between sites?
Anyway, I very much welcome opinions on this.