From the terms:

IF YOU CHOOSE TO PARTICIPATE IN THE GIFT EXCHANGE AND YOU DO NOT SEND A GIFT, YOUR TWITTER.COM USERNAME WILL BE POSTED PUBLICLY.

I didn't go any further, but didn't see this anywhere else but in the terms page. Are you going to run a list of users who do not follow through?

Probably far too strong choice of language. It's not something I plan on doing and I'm going to edit the terms to reflect.

Sort of a line to encourage social responsibility :)