In the back of my head was letting the user pick a charity and/or indeed letting the user specify a third friend to send it to (and maybe take a cut from there).
I'll keep reading what everyone here thinks and implement that once there's some consensus.
Having it go to the first friend encourages an obvious bias, although that just depends on how much you trust that friend. Having it all go to you, the site owner, seems a bit unfair (a massive reward/effort ratio!) and nominating a charity would be an excellent way of mitigating that. I've heard of a similar incentive-based scheme in which you pay money to an entity you dislike (e.g. democrat donating to republican party) if you fail.
1. Benevolent option: the money goes to the website but it goes back to the user after a year, less a fee (the website will also earn interest for that year so you'd get fee + interest income). Not only do you help people stop procrastinating but you also help them with a saving program.
2. Devious option (and probably more commercially viable). the website takes the money but the website also runs a weekly lottery that pays out some fraction of the revenue collected.
