Gmail was invite-only to join and get an @gmail.com account, but once you had it you could interact with any email account user, and they could interact with yo

Gmail was invite-only to join and get an @gmail.com account, but once you had it you could interact with any email account user, and they could interact with you. GMail isn't a walled garden. Facebook, Wave, G+, etc are. That's why they depend on rapid user growth very early on when the hype is fresh.