While I don't condone the actions, the part where you say "without telling you" is unfounded. I just went through the sign-up and there are clearly two checkboxes, which are checked by default: [x] "Share on Twitter [x] Follow @Prizes"

It's by Max Levchin's company Slide, which was acquired by Google last year. They have many apps that are popular on Facebook and were popular on MySpace.