There are numerous online experiments of webmasters who moved their websites from a subfolder to a subdomain resulting in a traffic drop. I think the issue may not be related to the subdomain itself but the 301 redirect. Since several years now, I have found there were many issues around 301 redirects and Google juice. It was like a webpage didn’t keep its rank after moving to another URL. Yes, I know Google says there is no problems around 301 redirects but we have pieces of evidence showing that it is not case. The blogspot & wordpress blog case:
Lot of people including me thought that subdomains were considered to be separated websites for Google. To illustrate this idea, I think about blogpost, wordpress.com and wix.com blogs that are hosted on different subdomains with the same root domain. I looked for cases where a wordpress.com, wix.com or blogspot.com subdomain has been removed from the Google index and I haven’t been able to find any reports online.
The Matt Cutts and John Mueller statements:
Then, Matt Cutts and John Mueller both said that subfolders were like subdomains from a Google point of view.
There is no real case showing that subdomains cause a traffic drop:
Our investigation about the iwantmyname.com case
The only example of a reported traffic drop that was reported in google search console is the iwantmyname case. After moving from iwantmyname.com/blog to blog.iwantmyname.com, the webmaster had to face a traffic drop from google.
However, the iwantmyname issue may not be related to the subdomain.
First of all, we looked into the code of blog.iwantmyname.com at web.archive.org and we didn’t find any rel=canonical tag in the blog.iwantmyname.com at the end of 2014.
There were also an issue with https. At the beginning of the year 2014, the domain was https://iwantmyname.com/blog. They moved the domain to http://blog.iwantmyname.com. The redirections may not have worked from HTTPs to HTTP. We found mixed content errors in the HTML code of iwantmyname.com at the beginning of 2014. There were internal A HREF links in HTTPS and HTTP. We think that Google was lost between HTTP and HTTPS. It is possible that Google considered http://blog.iwantmyname.com and https://blog.iwantmyname.com to be separate websites.
Beginning of 2014:
End of 2014:
I would like to warn you about the subdomain cases that you can see online. If the data result isn’t taken from google search console or google analytics, it means nothing to me. There are many SEO measurement software that provide “visibility index”. However, Those charts aren’t proven to be accurate if we compare them to Google analytics and Google search console.
The allegations against subdomains are not receivable. We do think that subdomains are as good as subfolder for SEO. In fact, subdomains can be very useful to host a website on another web server. For example a company can host its french website (fr.yourcompany.com) in France and the english one (uk.yourcompany.com) in the UK.