Building on someone's platform is a risk. But then again all new businesses have lots of risks, including the risk that they don't find enough customers for their product or service. I've hear about startups failing that way far more often.

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.