While true, I think it is important to emphasize the difference between 4 platforms and 4 carriers. With both postal mail and email, its not just that there are

While true, I think it is important to emphasize the difference between 4 platforms and 4 carriers. With both postal mail and email, its not just that there are alternative systems, but rather that you as the sender can choose any of the offerings to reach anyone, even if they use a different carrier for their own needs, so there is a lot more competitive pressure on those 4 carriers than with 4 platforms where network effects keep people locked in.