From Earth Day through May 21, Old Navy stores began accepting them in a specially marked bins inside stores. Your old flip flops will turn into material for future playgrounds thanks to a partnership between Old Navy and TerraCycle. The program is called Flip Flop Brigade, and you can learn more about it here.
Additionally, you can check out the TerraCycle site and learn how to participate in more brigades and recycle all kinds of things you didn't think possible--like yogurt containers, drink pouches and candy wrappers.
TerraCycle has already raised over $2 Million for charity.