Facebook sets a monthly challenge for you iphone shooters and has a dedicated page for this. Not really sure about what the prizes are but it sounds like a lot of fun. Here is the link and the rules are below: FACEBOOKIPHONECONTEST
The competition will last for 1 month at a time. Add your photo then right 'Vote' on other photos. The more votes will win the competition, as it's just a fun competition. There are no prizes, but you will be chosen to pick the theme for the next competition. Good luck! • iPhone Photos only!