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All of those offered, at the time they appeared, a completely new service that had essentially zero competition, and have become entrenched since then (can you even e.g. get a job without a phone number?). Not to mention that the USPS is mandated by law to exist, so it can hardly be compared with Facebook.

Facebook never offered anything so revolutionary. It's just a well put together implementation of something that was already possible.

I can send a letter or a package to my grandma via USPS without her having to pay any subscription to USPS or "sign up" for USPS in any way.

Now, the early days of the telephone system might have been a better comparison to use in your argument than USPS. But I think it could be easily argued that the early telephone offered something much more unique for it's time than Facebook does.

First class mail generates 51% of the USPS revenue today, and is projected to hold onto 35% by 2020. Not quite a "nominal fee", I'd think.