This goes for using any platform. Look at the apps developed using the Twitter platform. This is one reason I've been trying to work on other ideas for a business, or to do my own crawling, for unscatter.com. Using API's from another company is always going to be a product risk.
They all present massive opportunities to reach and interact with users/customers/clients... but unless you've been living under a rock for the last few years, you should know this kind of thing can and DOES happen.
This shouldn't and won't stop most people from building services on top of these platforms, but at this point you either know this is a risk and accept/plan for it, or you don't know this, which means you do not have an adequate understanding of the infrastructure you are depending on.
We had planned to use Facebook-only for the first phase of the app - as it has a prebuilt social graph, friend model, and notifications system. Easy to use, and with a large base of users.
I guess my point is that even if you think you have taken a calculated risk (90 days breaking changes), the companies can find a way to mess up your plans anyway!