Wouldn't you say that Android is currently the biggest competitor to the iPhone?
Apple & Google are both pretty big companies and will likely be competing in the same space given the number of products they both own. Apple's iAds are a direct competitor to Adsense on mobile devices, Apple TV/Google TV, etc.
Facebook does, at least in the sense that both operate in the web/user space, and both have advertising as their fundamental source of revenue.