> Spam needs censorship?
Spam needs to be deleted or hidden by someone or something or your inbox/feed will be full of spam. Trying to manage this individually with keyword filters or even the more sophisticated Bayesian anti-spam is exhausting.
> But isn’t spam only profitable at scale?
Yes. So what? Spam scales. It's like cryptocurrency, you burn trillions of FLOPS of computing power to make a few cents.
> Can’t you just charge a nominal fee or something?
Fee? For Facebook? This absolutely kills customer acquisition. It has often been suggested and never worked; the only site I think that does it is Metafilter?
Facebook went in the other direction with subsidised data plans that were free to access Facebook. The economics does not permit them to charge the eyeballs to look at Facebook. On the other hand, they can't afford to let people advertise for free on Facebook, since that's the service they do charge for.
In practice this is why accounts get tied to phone numbers so often, because it's slightly more difficult to acquire another few thousand phone numbers after the first thousand are banned for spamming, than it is with IP4 addresses.