Should I create my own community or partner with one?

One of the most frequently asked questions from "traditional / more established businesses" that wish to enter the web 2.0 space is whether they should create their own or partner with already established communities. We should explore the pros and cons of both options
I think there are also other options such as creating facebook apps or opensocial apps. It seems that eAcademy is exploring Opensocial implementation, so would be nice if I can hear some input from one of the panel.