(Sidenote: USPS actually is ad-supported by bulk mail, and "heavy users subsidized" by parcel post. First class letter postage is really a nominal fee to eliminate DOS and DDOS attacks. )
Facebook never offered anything so revolutionary. It's just a well put together implementation of something that was already possible.
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.