Google and its email system

When Google released its email service to the public(over a year ago) there was(and maybe still is) speculation that its particular way of signing up people only by invitation was a way for the company to generate interest in the service. i.e if one chap has a google email account he can send invitations to a limited number(5 or 6) of people to sign up, and those people can invite their friends and so on. The strategy, no doubt, was a success since gmail accounts are among the most coveted, for those who dont have it. Some days ago it struck me that there might be another logic behind google's move. There is a theory that every person in the world is "connected" to every other person my atmost 5(or 7) people. i.e if you pick 5(or 7) of yours friends and then they pick 5(or 7) of their friends and if the process is continued, then you might "reach" all the known people in the world. I dont know how far this theory is true but it seems to me that google may have wanted to test this or believing in it relied upon it.