That's not just GMail, that's all compliant email handlers; it's part of the RFC. Unfortunately, it's not guaranteed that the site you're signing up to can handle that address format - Facebook doesn't...
While the RFC mandates the ability to send and receive email from user+detail@example.org it doesn't require that it go to user@example.org. It's an extension to the email standard, and you could have labels in another form if you wanted to, e.g. inbox0-user@example.org (http://tools.ietf.org/html/rfc5233)
