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Can anyone explain why one is better than the other, and in what circumstances? Things I'm considering: SEO consequences (if any), scaling issues (if any) and anything else I'm not thinking about.

Facebook, for example, uses subdirectories for profile pages. Tumblr uses subdomains. Are there specific reasons why?

Another consideration: domain mapping.

Hope the question isn't too general, but I haven't found a clear answer.


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One reason to choose subdomains if you're looking to add authority to the main site: [link]wordpress.org/support/topic/… – jw60660 Oct 3 '11 at 22:01

First, the link for two gems for every Multisite expert or wannabe or in-between.

WordPress Multisite 101
Setting up a new WordPress install is pretty easy. Setting up Multisite requires just a little more savvy, and the information is all over the place. This ebook strives to pull it all together, explain you what skills you need to get started, and move you to the next step: running your own Network. Think of it as a basic tutorial in running your own Multisite.

WordPress Multisite 110
While it’s an extension of WordPress Multisite 101, 110 isn’t a supplemental, in fact it’s longer than 101! This is the next step for people who’ve managed to get the site up and running, but want to do a little more. In this ebook, there’s more philosophy and discussion, and fewer direct answers, because now you have to make decisions based on the pros and cons. Are you ready?

These free1 e-books are written by no less than two giants of Multisite: Mika Epstein (aka Ipstenu) and Andrea Rennick.

Here's the snippet from the 101 that answers your question:

Subdomains vs Subfolders

A subdomain install means that your sites will look like this:

  • Main site:
  • Sub Site #1:
  • Sub Site #2:

A subfolder install means your sites will look like this:

In addition, if you use a subfolder install, the post URLs on your main site will have the slug /blog/ prepended to them, causing them to look like this:
At this time, you cannot customize that slug, nor easily remove it.

There is no SEO benefit at this time to using subdomains or subfolders, and you can map a domain name to either install, so the decision is purely cosmetic. See http://www.mattcutts.com/blog/subdomains-and-subdirectories/ for an explanation

The Domain Mapping subject is extensively covered, so I'm pretty sure that the 101 reading will answer this and many other questions you may have.

1 Lot's of donation options in the e-book pages, choose the best that fits :)

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Scaling really isn't an issue either. The only gotcha I've run into is when someone on the main site made a page with the same name as a subsite on a subfolder install. Easy enough to fix, and caused much giggles. - My general thought is 'If all the sites are 'the same' then it's subfoldrs, if they all look different, subdomains.' – Ipstenu Jun 3 '12 at 15:24
Here we go, the gap I left in the answer filled by the half-elf support rogue herself, thanks @Ipstenu ! – brasofilo Jun 3 '12 at 20:05

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