There are 'big guys' like Wikipedia who use a subdomain, and some big guys like the sites which you have mentioned who do not use a subdomain.
In some sites, like Wikipedia, being multilingual means offering the actual content in different languages. Because it is different content, it makes sense to keep the several sites separated.
In some other sites, like Twitter or Facebook, the content is the same, regardless of the language of the user. Only a few elements should be addapted to the user language, such as the controls and menus.
It is important to keep the language away from the content, but when the content is the actual object to translate, another version of the site, more than just the menus makes sense.