As I know,
mail.google.com all comes under the same domain and its all are subdomain. Entire web treats these subdomain as the part of main domain and they have same Alexa rank, PageRank and all.
But in another hand, take a look on
webs.com; these are different sites but blogs or websites under those domains are treated as different sites.
Its new URL, all have different PageRank and Alexa rank as well.
Tts about millions of subdomains under those few domain, have almost similar IP address, hosting and CMS, still why they are called different domains?