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I went through a great deal of stress thinking about the same thing as you - I have several great domains that complement my main domain very well. Considering that they are industry terms, they would bolster what I do even more. I went through every possible analysis and came to this conclusion and hope it helps you out.

**1) If it's about your brand, then it's about your brand. **

Sounds simple and stupid but it's true. Taking each domain separately is nice but people were always most impressed with the main one. And at the end of the day, when they wanted to easily find everything, they expected it to be on my main domain. They expected to interact with me at my primary domain. They did not expect to take out accounts on three web sites and write to me at three email addresses.

2) Domain authority issue with SEO. This has already been mentioned so I won't expound except to say less is less and dividing what you have elsewhere makes you a small fish in a smaller niche rather than a larger fish in a bigger sea.

3) Managing 1 site is much easier than integrating 4. Subdomains are a pain to manage more than subdirectories. It only gets more challenging when you're moving to completely separate domains, may want to deal with one single database for all, firewalls, etc. It's yet another hassle you may not need or want to deal with.

So what am I doing? I'm going to set up little silo sites on each of those domains and primarily use them as doorways into my primary domain. They will have RSS feeds from our main sites, some other rss feeds from some other locations and might actually attract some unique content there as well. Nice to have but my primary focus is on my main domain.

And after reading dozens of posts on this issue, I'm very glad to have made a decision and moving in that direction. If I ever want to move my content to those domains, I'll do just that when the time comes. Best of luck to you.

My question is mostly directed about the impact on search engines, not about clickthrough ratios, etc. although I appreciate the comments. I've decided to potentially offer a submenu on the top horizontal area instead of optionally placing it on the side, which may not be as obvious. Some say it's fine there but let's say the submenu seemed like a good offering. The issue is whether all those links placed before content will have an effect on SEO. I have seen repeatedly discussions about how having your most important content up at the top as high as possible is most important. Having this many links above the main content is a big question mark - and that's the question here:

Q: Since a submenu will have at least 70+ links in it, is it an SEO liability if it is placed before the main content on a page loads?

I have an ecommerce shop here: http://m00.biz/UHuGGC

I've added a submenu for each major category and subcategory of items for sale. There are over 60 categories on that submenu. I've heard that loading this (and the number of links) before the content is very bad for SEO. Some will place the menu below the content and use absolute positioning to put the menu where it currently is now. It's a bit ridiculous in doing things backwards and wondering if search engines really don't understand. So the question is twofold:

(1) Are the links better in a bottom loading sidemenu where they are now?

(2) Given the number of links (about 80 in total with all categories and subcategories), is it bad to have the sidemenu show the subcategories which, in this instance, are somewhat important? Should I just go for the drilldown, e.g. show only categories and then show subcategories after?

Truth is that users probably would prefer the dropdown with all the categories and second level subcategories, despite the link number and placement.