I've thought of a novel way to spread Giffgaff... When GG send you out new sims to share they can also send some Giffagff balloons. You pop the sim card into an envelope and attach it to the balloon (once you've filled it with helium). A note in the envelope says something along the lines of,
"This is part of a social experiment. Please register your details on www.giffgaff.com and let us know where you are and how you got this balloon and who the users name is. As thanks for doing this, register the attached sim card and recieve £5 worth of airtime credit!".
And then you release the balloon from your house. We will see how far the balloons got and maybe a prize could be offered to the user who gets the furthest away from home.