Their financial model is similar to a TV or newspaper.
Imagine you create a newspaper, you start out showing no ads, lots of people buy it, it is all good.
Then you raise money from investors to grow, and they start asking for more revenue. So you add one advertisement.
Then the investors want out, so you have to IPO...more ads. Then you have to keep the share price up because execs want options, and your HR consultant tells you that the "top" execs need to see a rising share price (as if you need a "top" exec to add more ads to a webpage)...more ads.
The ability to add more advertisements is limited by the space in the newspaper, but there is almost no marginal cost associated with adding more ads. So it looks like very clean growth, very strong economics.
This is basically how Google and Facebook work. Their revenue growth looks puzzling because they seem to achieve very consistent, very predictable revenue growth with no extra resources. The reason why is that they can add more ads to a page very easily, and can control their level of revenue growth so they can keep employees/investors happy (in that order). It is true that they have also achieved some pricing growth but that advantage has been capped (and is largely a function of their monopolistic position, i.e. Doubleclick allows Google to play both sides). Most of their growth has been increasing ads per page.
There is no way for them to function as public companies in this environment (again, I would say the situation with employees is actually taken more seriously than investors) without making their product worse. They won't stop before the product collapses.