On November 16th, Meredith Whittaker, President of Signal, published a detailed breakdown of the popular encrypted messaging app’s running costs for the very first time. The unprecedented disclosure’s motivation was simple - the platform is rapidly running out of money, and in dire need of donations to stay afloat. Unmentioned by Whittaker, this budget shortfall results in large part due to the US intelligence community, which lavishly financed Signal’s creation and maintenance over several years, severing its support for the app.
You’re right, the problem with something like proof-of-work is that it doesn’t prevent abuse at scale. With a sufficiently powerful computer, the same bad actor could crank out hundreds or thousands of phone numbers with the only variable being time and energy it takes to run their computer. (And the computer would only need to do something that takes a phone, even a low end phone, maybe a minute at most to do.)
At the very least, obfuscating phone numbers and allowing users to share usernames would be seriously preferential. I’m okay with Signal having some record of my phone number, but not so okay with handing it out to other people willy-nilly.
You could run a network like signal, and either charge a small amount of money per message, or a larger amount of money to register with the network.
Hell you could do the WhatsApp model, charge a dollar for new users, the pay for the registration verification. The same thing.
You just need some mechanism to add friction for mass spamming, be that money time or complexity.