One Million People (from the guy who made the Million Dollar Homepage) 16 years ago
A quick idea that could avoid the "why bother" and originality issues.
There is a 25m x 4m swimming pool here, with about half a million 2cm x 2cm blue tiles (http://imgur.com/UdzbE.jpg). Imagine that for a cost of printing a tile, one could put a tiny picture of their face in this pool.
Why would anyone do that? This is a 5+ star mountain resort located half a mile from several 2014 Winter Olympics sites. Those who play the tile game shall get a lifetime discount on resort's accommodation and services.
Advantages are simple: participants get discounts, those who stay here have a more fun swimming experience, and the resort gets clients and promotion (even if the effort is unsuccessful in the end).
Disclaimer: I'm not affiliated w. the resort.