Isn't iOS7 for iPhone 4 and greater? Isn't that like 80% or more of the iPhone market?
From Wikipedia:
AirDrop, officially, is limited to these models (not supported on earlier devices because of Apple's policy) and newer:
Running iOS 7 or newer:
iPhone 5
iPhone 5s
iPhone 5c
iPad (4th generation)
iPad Mini
iPod Touch (5th generation)