what does it mean to call OCLC a monopoly?

Karen Coyle has a new blog post detailing what Sky River’s specific allegations against OCLC are.

[O]ne could look on WorldCat as a shared community resource, not the property of OCLC. In fact, OCLC uses this kind of argument in its record use policy, but somehow leads to the conclusion that WorldCat should not be used to foster non-OCLC library services. It seems easy to make the opposite argument, which would be that WorldCat could be the basis for a wide range of services that would benefit libraries, even if they do not come from OCLC.