More importantly though, the trend is that most companies relying on someone else's platform for survival are cannibalized. A few examples to the contrary don't make it a good idea.
Lots of companies make tons of money building on platforms like facebook, the iOS App Store, and so on. The question was never if that was a risk free move (nothing is), by whether the potential gain outweigh the risks.
However in the real world where there are very few product ideas that succeed, it is not a bad idea to start something that you feel has a good market, but relies on another company's platform. because the alternative is waiting decades to start a perfect idea.