It very important that anyone reading this thread and thinking 'maybe this is good, I'm new, and it sounds like plenty of exposure' - well you are kidding yourselves.
As a professional photographer, I've done work in the past that has been paid and credited, and I have received absolutely nothing back from those credits, no work and not even any real enquiries.
However I get tons of work through word of mouth.
A year or so ago, purely as an experiment I shot for a music artist with 1.2m followers on facebook, and loads of other social media presence. I made it part of the deal that I was heavily credited everywhere with these photos. You know what? nothing. I didn't even see a spike of traffic to my website.