It should be easy: 10% of revenue from fraudulent ads? Fines amounting to 15% of the total revenue. This way, Meta will be incentivized to invest ~5% of its revenue on getting rid of that 10%.
The USPS is like this because of the persistent belief that it's not enough for government entities (think USPS, Amtrak, etc) to provide a good service for the citizens - they must also (try to) turn a profit.
If we as a society considered it acceptable for the USPS to spend money to ensure everyone in the US had mail access without selling out to corporations to turn a profit, they wouldn't need to have products like EDDM blasting spam to entire zip codes.
But I suspect you mean to say 90% of what ends up in your mailbox is junk. If that's the case, there's a fix for that, too: