It seems like cheating/fraud to me. Dropbox provides a service that usually costs money. They're also offering to provide service for recruiting new users instead of charging money.

If you fake users, you're getting the service without fulfilling your side of the bargain (paying money or recruiting users). That seems like fraud to me.

That being said, I'm sure Dropbox expected some of this behavior and is still happy with the legitimate signups and overall press they're getting.