Dropbox has traditionally used storage space as an incentive for referrals and beta testing, and now it's sweetening the pot. The personal cloud data storage company has just doubled the amount of space awarded for each successful referral to 500MB, up to a maximum of 16GB, and users with paid accounts will now receive 1GB per referral up to a maximum of 32GB. If there's anybody you haven't invited to Dropbox already, now's the time to work your social magic.