Marketing. Social media is the most powerful tool for marketing whether you like it or not, and by hosting a fan contest on a social media site it is essentially low-cost/free marketing for them. "Show off your Au Ra!" might look like a cute competition to us, but of course there is the business and marketing aspect of it for SE, with the whole idea of getting these images trending and circulating the web. It makes the people playing the game do the marketing for them. If the contest is hosted on the forum, it's not exactly being marketed to a new audience.