I'm up to 11GB free, it's possible. I've sent them over 150 referrals, most of whom've completed signup. Plus doing things like taking the tutorial, etc.

The referral model by dropbox must be awesome for customer acquisition.

150 referrals, wow. Do you experience some kind of saturation by now, in that it becomes increasingly harder to find a potential new user who hasn't yet subscribed?

actually I don't put any work into getting referrals at all, so I don't know. I wrote a blog post about it and put the link on twitter a few times, and the referrals just keep pouring in - I'd love to know where they all come from to be honest! I've hit the limit as to my free space allowance now.