Harb got me on to this a couple weeks ago and I've been spamming friends with the keys occasionally.
The Golden Key (SHiFT) codes are put up on the Twitter page of Randy Pitchford every 10hrs or so, a few days a week.
They are only good for one redemption per user; but thousands of unique redemptions in total. I've found through trial and error that they generally last up to 2hrs before all the redemptions are used up.
https://twitter.com/DuvalMagic