Bryan Caplan asks where monopoly power comes from. I would follow this with another question: is the standard textbook account a result of intellectual path dependence--the hangover of 1950s-ish theories of market power combined with the fact that it's easy to teach and test? Here are a couple of links of interest:

1. A claim that MySpace is a natural monopoly.

2. A claim that Facebook is a natural monopoly.

3. A claim that Twitter is a natural monopoly.

4. Google search results for The Boy Who Cried Wolf.

5. Discussion of whether the DOJ will go after Google; after all, they're a natural monopoly.