* The fine, an FTC record-breaker, is equivalent to the cost of simultaneous acquisitions of multiple high-profile startups.
* A mid-single-digits repetition of this fine would deplete Facebook's cash reserves entirely.
* The fine covers specific conduct committed by Facebook (violations of their previous consent decree) that almost certainly netted the company multiple orders of magnitude less money than the fine cost them.
* The fine was anticipated long before it was announced and has been priced into Facebook's stock; we can't look at the spot price now and know what it would have been without the fine factored in, because that's not how stock prices work.
The purpose of the fine wasn't to punish Facebook for being Facebook; much as I might want that to happen, FTC doesn't have the authority to do that, nor should it, since it's an unelected administrative apparatus without lawmaking power. The point is to deter future violations of this and similar consent decrees at Facebook and other companies (Facebook being the largest of that cohort of companies). It surely accomplished that: whatever money Facebook made off of Cambridge Analytica and similar enterprises is surely dwarfed by the $5B fine, since chicanery like that isn't Facebook's core business.
In the US, when a wealthy person parks in front of a fire hydrant, we don't give them a $100,000 parking ticket; the fine is scaled to the offense, not the offender.