Charles Miller explains why Gmail’s system of handing out invitations is a good way to carefully scale up the system’s capacity instead of just opening the floodgates to everyone at once. What I haven’t seen mentioned anywhere is that Gmail’s invitation system seems to be roughly modeled by Google’s earlier efforts with Orkut, the invitation-only social networking system. With Orkut you could invite as many people as you wanted, but there’s not nearly so much demand for Orkut as there is for Gmail.