Economics
A network's value grows roughly with the square of its users, not in a straight line
Robert Metcalfe argued in 1980 that a network's value scales with roughly the square of its connected users, since each new member can potentially reach everyone already on it — economist George Gilder later dubbed this 'Metcalfe's law.' It's why a single fax machine was worthless and a million of them were indispensable.