Communication is very much important in the project, the communication management plan is set up so that there is proper communication within the project team as well as with the stakeholders.
The project manager should do all the communication between the project team as he is the one that is responsible for the overall project also he should communicate with the stakeholders with the insights of the project.
The platform to be used is totally dependent on the project team, project managers and the stakeholders whatever they all agree upon, it could be E-mail, companies own integrated messaging system or any other communication medium they decide on, the important part is there should be communication.