You might have heard that we launched Facebook Stack Overflow yesterday.

As part of this, we modified our code to put <meta rel="canonical" ... tags on every question and user on the facebook.stackoverflow.com domain that points to "vanilla" Stack Overflow.

For example:

iAd error "Ad inventory unavailable" on facebook.stackoverflow.com
iAd error "Ad inventory unavailable" on stackoverflow.com

On facebook.stackoverflow the html contains the meta tag

<link rel="canonical" href="https://stackoverflow.com/questions/3720459/iad-error-ad-inventory-unavailable">

The intent there is to tell Google "these are exactly the same page, impart all page rank to Stack Overflow's copy and prefer it in search results".

This seems like the point of rel="canonical".

A canonical page is the preferred version of a set of pages with highly similar content.

It's common for a site to have several pages listing the same set of products. For example, one page might display products sorted in alphabetical order, while other pages display the same products listed by price or by rating. For example:

If Google knows that these pages have the same content, we may index only one version for our search results. Our algorithms select the page we think best answers the user's query. Now, however, users can specify a canonical page to search engines by adding a element with the attribute rel="canonical" to the section of the non-canonical version of the page. Adding this link and attribute lets site owners identify sets of identical content and suggest to Google: "Of all these pages with identical content, this page is the most useful. Please prioritize it in search results."

However, we're seeing Facebook Stack Overflow results and sometimes they even outrank vanilla Stack Overflow (an example). Maybe this is something to do with having an independent sitemap.xml for facebook.stackoverflow.com (sort of a shot in the dark)?

example Google search result

So, what are we doing wrong here?

We're kind of hoping to keep searches of the form site:facebook.stackoverflow.com working, but giving those up is totally acceptable if a total rel="noindex" is required.

2 Answers 2


According the same page you linked to rel="canonical" is only a suggestion and not a directive:

Is rel="canonical" a suggestion or a directive?

This new option lets site owners suggest the version of a page that Google should treat as canonical. Google will take this into account, in conjunction with other signals, when determining which URL sets contain identical content, and calculating the most relevant of these pages to display in search results.

It's possible that Google simply has chosen to ignore the canonical URL or has yet to act on it.

But the reason why it outranks the StackOverflow question is probably simple: (assuming that Google is ignoring the rel="canonical" directive) the Facebook subdomain has "facebook" in the URL which carries quite a bit of weight in Google's ranking algorithm.

  • We were hoping canonical would compensate for that... I guess not. The related issue of FB.SO results showing up on a page even without a facebook search term (example) does sound like Google is just ignoring the suggestion. Aug 26, 2011 at 18:55
  • 1
    Google seemed to work with SO on the duplicate content issue where other sites were outranking SO for SO's original content. I wonder if they would look into this for SO as well?
    – John Conde
    Aug 26, 2011 at 19:25

I see that this issue is sorting itself out now. You can test by doing an info: query on Google like this which brings up the new URL.

google result for facebook.stackoverflow.com/q/3720459

The thing about the canonical tag is that it’s a directive which means it doesn’t work 100% of the time. 301 redirects are the same – they don’t always pass 100% of the weight through the redirect.

Also with either of them it takes a few weeks for Google to understand that they are there to stay and should be followed. This is an issue with new pages because they go live and will be on the wrong URL for a few days/weeks before Google follows the canonical.

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