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It is possible that the social web will end up with a larger number of federated instances as opposed to the single dominant player approach we have at the moment. Facebook may be the largest social network, but it is far from the only one. Increasing specialization and the desire for interoperability will encourage federation.
The mobile or cell phone market is a good example of what might happen. At first, it was hard to place a call from one network to another. Consumer demand led to this becoming much easier. Now it is not even noted as an issue—any mobile phone in the world can call any other phone.