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They also had the element of scarcity, not only did you require an invite, but those invites themselves were very hard to come by. USB's were traded for them, and sites dedicated to getting them, gmailswap.com anyone?

Invite-only has been pretty successful at Quora (now public) and Forrst so far. A lot of designers also want in on Dribbble because of the scarcity of invites they've created.

Google does not do this as a technique of attracting interest, they do it so they can scale more predictably. If it increases interest, it is only because people already wanted it.

Only because Google did it.