As Buth Bebo mentions,
The economic logic behind these positions is a straightforward application of Metcalf’s law, which states that the value of a network is the square of the number of nodes in the network*. A corollary to Metcalf’s law is that when two networks connect or interoperate the smaller network benefits more than the larger network does. If network A has 10 users then according to Metcalf’s law its “value” is 100 (10*10). If network B has 20 users than it’s value is 400 (20*20). If they interoperate, network A gains 400 in value but network B only gains 100 in value. Interoperating is generally good for end users, but assuming the two networks are directly competitive – one’s gain is the other’s loss – the larger network loses.
I liked this analysis as for what would happen if Google Me becomes a success…

