We use each level of communication for different reasons. (is it a quick question? Is it a well thought out idea that needs attention? Is it time sensitive?) It feels very natural and after a short period your organization falls into a groove as long as Basecamp is the ONLY method for work communication outside conference calls, etc.

Do you have any formal guidelines of how to communicate when and which method? Or do you just figure it out organically?

Planning to move everything to Basecamp. Having discussions in multiple places makes it difficult to track at a later point.