Aside from the fact that they are giving away free gift cards, it's actually a pretty cool site. You might want to check out some of the other deals that they are offering. Lots of great half of deals!
Also, make sure that you and your friends click on the confirmation email you will receive AFTER your sign up. If you don't confirm, you don't earn referrals. If your referrals don't confirm, they don't count!
As always, Enjoy!