Sometimes webmasters have very strong opinions as to whether subdomains or subdirectores should be used in organizing a website. Naturally, the preferred or more preferable method depends on the particular circumstances of each site, as well as personal inclination. For example, this site is organized into two subdomains (en.wasaweb.net and ja.wasaweb.net), because I treat the English and Japanese versions as essentially two separate sites, only sharing the same domain name (wasaweb.net). In other words, each language version is independent from the other, not subordinate to the other, and they do not and should not mix. Previously, the two language versions were in subdirectories as in www.wasaweb.net/en/ and www.wasaweb.net/ja/, however, as they were considered the same site, it was impossible to set different languages for third-party plug-ins.
There are pros and cons for using subdomains or subdirectories. There are many factors to consider, and in this piece, I would like to touch on one such factor: Google+ pages. A Google+ page is the public identity and presence on Google+ for non-person entities, such as businesses, organizations, and corporations. One advantage for setting up a Google+ page is that the Google+ page can become the publisher for a website. What does that mean? It basically, and for lack of a better, clearer expression, ties in the website to the Google+ landscape. By using the publisher mark-up, it is possible to establish a bi-directional link between the Google+ page and the website. This results in, for example, the number next to the +1 button to synchronize between the Google+ page and the website, so when someone +1s the Google+ page, the count on the linked page (the home page) also increases, and vice versa. There may be more benefits in establishing a presence on Google+ in the form of a Google+ page, and linking it to a site, such as appearance or increased prominence in SPYW (Search, plus Your World) or the Knowledge Graph. It would not be surprising to see such benefits to grow in the future.
Currently, a Google+ page can be the publisher for one site only. Google+ considers subdomains on the same domain as different sites. If, for example, a site is organized into three subdomains, www.example.com for desktop visitors, m.example.com for mobile visitors, and blog.example.com for the site's blog, then these are considered by Google+ to be three separate sites, and for publisher mark-up to be established for these subdomains, it will require three distinct Google+ pages. In some cases, these three subdomains would be sufficiently distinct from and independent of each other, therefore creating three Google+ pages for each subdomain makes sense. Other times, and it is probably likelier, such would be duplicating or triplicating the effort and the same content, thus potentially diluting the strength of the Google+ page, since the three Google+ pages will be splitting the followers, which could have been usefully collected on a single Google+ page. It follows that, and as far as Google+ pages and the publisher mark-up are concerned, it would make sense to be clear whether something is an integral part of the business's or organization's website therefore organize them as subdirectories on the same (sub)domain, or it is independent of other content and as such makes sense to organize them in subdomains. In this site's case, there are separate Google+ pages, one for the English version of the site, and another for the Japanese version of the site.
Google+ is a rapidly changing product, and for that reason, I would hesitate to overhaul the site's structure solely to accommodate Google+ and the publisher mark-up. However, it would perhaps be worth something to bear in mind, when restructuring an existing site, or setting up a new site.
Google+ Business: www.google.com/+/business/
About Google+ Pages: support.google.com/
Google+ Pages: Learn how to link a Google+ page to your website: support.google.com/
Search, plus Your World: www.google.com/
Knowledge Graph: www.google.com/