It pays to read the fine print.
Even if you delete your photographs from sites such as Facebook, they may still have the right to publish them without your permission long after you have taken them off the web. Apparently, in February 2009, Facebook changed its terms and conditions. You might want to read the fine print before you post any more photos on their site.
This same caution might need to be exercised when entering photo contests. Be sure to check and see who retains the copyright and that you are not giving a royalty-free license to distribute your image. Look for wording such as 'Copyright in all images submitted for this competition remains with the respective entrants'.
Pro-imaging (http://www.pro-imaging.org/) looks at the terms and conditions of various photo competitions and rates them as good or bad.
Article from Digital Photographer, Issue 82, page 10-11.