As a business, it will always be a bad idea to completely rely on another entity outside your control, whose interests are very likely to be misaligned to yours. By all means leverage, Facebook (and Twitter and others) sign on and APIs, but if you are building a business on a single one of those, you are completely at the mercy of their whims, any incompetency, and changes in business practices.
You do not rely on other companies only when you rely on open standards. Web development is an example (although you indirectly rely on Google for example). All kinds of apps usually rely on an underlying commercial technology ( windows development, osx development, ios development) whose actions are outside your control.
huh? how do you figure that?