Rather than revenue diversification, I was thinking more about product diversification. With core products like GMail, search, Plus (hah), and Google Apps, if any one of those takes a catastrophic plummet in traffic, their revenue wouldn't crash completely. In their early days, if a competitor took over search, they would have no remaining products that continued to generate revenue.
Because Google is an advertising company (so is Facebook). It shouldn't be a surprise that advertising is their main source of income. But at least you can point to a multitude of products within which Google places ads, as well as devices and software to provide places to put ads. Facebook, not so much. Yet?