This has been tried before - and failed (Score 2)

A number of apps have tried this in the past - Disa is one that springs to mind. As soon as it became popular, WhatsApp and Facebook stopped their services working with it. Even the services that did work were severely limited, meaning that you had to go to the actual platform to do anything other than text. The whole point of these apps is to lock you into their eco-system, and they have no incentive to let another app sit in the middle.