1. Product
Facebook is basically a giant directory. Google is a search engine.
Search engines are necessary to get any information anywhere. While Google doesn't have as much of a monopoly on this (i.e. Bing, Baidu, DDG), it is still the best by far and this alone is invaluable.
2. Side products
Google has a lot of other killer side products - Waze, YouTube, self driving cars, Android. Facebook has... React, WhatsApp, and Instagram. Just Android alone gives them more power than Facebook.
3. Productivity
Google regularly pushes the envelope on workplace innovation - 10% time, healthy catering, meditation.
Facebook has open offices and they work on one of the most distracting websites in the world. They are no contenders to the legendary Bell Labs. In a race, I would say that Google will win.
4. AI
AI will likely be the next big thing. Both are doing well in AI, but Google has been doing it longer. Google also has more important data and a broader perspective of how it all links together.
To give more context: Truly huge companies like Google or Facebook are actually conglomerates of thousands of informal tiny companies, that often are their main competition. Look for example to the large number of projects that Google created or companies it bought and then closed two years later. Each time there was people behind those projects, for them it was a huge stress when their project was dropped. This way to manage companies, helps their board to control competition by internalizing it.
Think "The Soul of a New Machine" by Tracy Kidder on steroids.
Google revenues can be viewed in several ways, in one of them it is not diversified at all as in 2016, 71% of their revenues came from Googles sites.
In another interpretation, they are very diversified, as this 71% revenue stream comes from a set of diverse sources. In yet another interpretation they are quite diversified as the USA provides quite half of their revenues.
Accounting could tell very different stories with the same numbers.
For the board, if the revenues are steadily increasing it shows they understand their world, which is not a trivial thing at all. It is basically the difference between strategy and post mortem analysis. One is easier to do than the other.
Google, meanwhile, has enough products and services to keep going far into the future. However, I do expect them to continue to grow slower and more into a sort of IBM for the internet.
Google has a lot of self-generated value which would take a lot more effort to compete with, as we've seen many google killers have come and gone with claims of superiority only to have their offerings revealed as lacking in comparison. Again if something shinier game along that Google was unequipped to handle, you might see a fork in preferences, and then opportunities for others start to arise.