But those that do use Gmail are well aware that they can use their Gmail account to write to @anywhere.com, not just other @gmail.com addresses. They already ha

But those that do use Gmail are well aware that they can use their Gmail account to write to @anywhere.com, not just other @gmail.com addresses. They already have that mental image in their head. This is "just" extending that concept to @username@twitter.com, @username@facebook.com, whatever. This user on this site, same as this email address on this domain.