How about this. Giffgaff is innovative and enterprising!
For PayG users, let them only pay data charges for websites not on a personalised website list " I use Facebook everyday on my mobile. If i paid 20p a month (say), i will not get charged for going on Facebook on my mobile, but i will get charged for everything else"
"I use Twitter and Flickr alot on my mobile alot, and if i pay 25p i can go on these websites for free, no data charges. All others i get paid for"