Stating a truism - to make a billion dollars, you either have to get $10 from 100M sales, $10k from 100k sales or $10M from 100 sales. Although each option leads to the same revenue, there are major implications as for the amount of support and attention you can spend on each customer.
Google/Facebook/Twitter obviously run the "$10 from 100M sales" model - meaning the only way they can provide profitable support or moderation is via inanimate algorithms, and deal with the PR fallout when they go wrong.