An incentive may not be required, but it definitely helps.

For example: I joined PayPal 10 years ago because my friend coaxed me into it. PayPal paid me $5 for joining and paid my friend $5 for inviting me. We invited lots of people..

On the other hand, consider GMail. It was just so massively superior to anything else out there that as soon as I heard about it I was hooked. GMail was (and is) free, but back then people were actually paying for invites..