Between all the people who use Android, Chrome, gmail etc, Google would be just fine if Apple and Mozilla changed default search engines.
At the moment, Facebook are a one trick pony that many people can easily live without.
In fact, it's probably Android that has put them over the edge. Gmail still has a fragile position. At least as far as my browsing habits are concerned, using Gmail didn't lead me back to google search or any of their other services.
Could you imagine what the world would look like if somehow in 4 or 5 years Firefox OS ended up eating Android's lunch?
Search is a major component of their advertising business, which these days is their core business. If people switched they'd definitely be in a major pinch.
80%+ of people who own smartphones use android, and they have phones for most niches - big, small, cheap, stylus, unlockable, replaceable battery etc. They have several manufacturers making phones with it. Their position there is not fragile either.
I'd love to see Firefox take off, but there is no way it is going to be bigger than Android.
Sure, there is a very small chance that everyone will stop using Google's products, but the chances of people moving off Facebook is much greater.
There is always some company that's going to eat the dominant company's lunch.
To think otherwise is to ignore history.
And I don't remember anyone saying that people wouldn't eventually move away from iPhones, blackberries and Palm pilots. People might move away from Android, but it would take many years, and it probably won't be en masse to FFOS.