The other mistake people make is the lake of engagement which ironically is what social media is all about. Rather than just collect my email, solicit my ideas, opinions, etc. Sure I’d like to win a new iPad but I’d also like a chance to earn that iPad by naming your new product or finding Waldo, or…well you get the idea.
The most successful social media contests I’ve seen are the ones that cross promote with other mediums. A mistake many people make is to invest all the time and resources in a social media contest in just social media when they need to piggyback it with traditional mediums like email, direct mail, broadcast, etc. Make social media the star not the only player.