You might have noticed that whenever Google launched its new product or a new version of its product, it provides a limited access to the users by providing only few invitation over your account if you have already got an invitation from the Google Team. Whenever they launched its product like Gmail, Orkut new version and Google Wave, it only gave out few amount of invitation to some limited members.
If you see that thing deeply, they are indirectly giving access to everyone and mainly the Active Users. Google always have a good and innovative ideas roaming around its head... As, its product are of excellent quality for works as a result most of the web users are in need of its product.
Why they put forward the new user joining through invitation?
Its really an interesting topic on which everyone is discussing. Actually, if i ask you to give out 4-pens to some of your friends and you have more then 30-friends standing and out of these 30-friends only 5-friends are good in studies, then now whom are you going to give out the 4 extra pens..?? Probably you will be giving those pens to any the top 4 of those good friends who can make the value use of those pens.
The same way the Google Works, as it gives you limited invitation to invite your friends and naturally you will be choosing those of your friends who are active and can make a value use of it.
Google works like viral invitation and chooses out the 90 percent of active users.
How Google forms a viral Chains of users?
Hmmm... You are surely thinking of why i called it a viral invitation! Actually when you got invited from Google you got around limited invitation for example. New Orkut Version is providing only 4-invitation and you send out the all four invitation to your friends and they too got the same four invitation each. They again send the four invitation , this way the invitation goes on... If you calculate then you will find : 4x4=16 again 16x4=64... Can you believe that from your chain, you indirectly invited sixty four friends.
So, anyway i hope you got the idea why Google always choose the path of limited before its product is exposed on the public.