Canonical has launched the Ubuntu One referrals program for ubuntu one users.The program is very simple,Just send the referrals link via email or Twitter and Facebook.And each person you invite who signs up for an Ubuntu One account will get 500MB of extra free storage and the signee will also get 500MB of extra free cloud storage.
The limit is Maximum of 40 people and you will get extra 20GB of cloud storage free.
Ubuntu One