Apple customers are famously loyal to the brand. But deep down, a lot of them may be just practical. They know Apple's products will all work well together. That single attribute has helped Apple defend its turf from hordes of would-be competitors. Some liken it to the network effect that has made social media firms like Facebook so popular.  

A new generation of hardware startups is hoping to benefit from a more direct version of network effects, by devising ways to make their products more useful the more people use them.