Though it wasn't explicitly listed in Marc Andreessen's "onion theory of risk", platform risk can be one of the most existential risks for a startup. Properly understood, "platform risk" is the risk that you incur when you build on top of rails that you believe to be credibly neutral.
If you are a farmer in the middle of nowhere and the BNSF railroad is the only way you have to get your produce to a city, you have platform risk. If you are an e-commerce company and the vast majority of your user acquisition comes from Facebook, you have platform risk. If you are the President of the United States and your primary outlet to reach people bans your account, you have platform risk.
Reading "No Filter: The Inside Story of Instagram" there was another great example of platform risk: shutting off access to a follower graph:
Twitter has confirmed the removal of its follow graph data from Instagram, disabling the "Find Twitter Friends" feature in the photo app. Users can still find people from their phone's contact list and Facebook friends list. "We understand that there's great value associated with Twitter's follow graph data, and we can confirm that it is no longer available within Instagram," Twitter spokeswoman Carolyn Penner told Mashable. – "Twitter Confirms Removing Follow Graph From Instagram's 'Find Friends'"
Luckily for Instagram, they had already piggy-backed off Twitter's follower graph which is one of the myriad reasons they were able to grow so quickly. While massively impressive, whether their growth was truly organic is an open question. In any event, when we talk about platform risk in the digital era, Twitter cutting off Instagram's access to their social graph ought to be top of mind.