Free: It’s one of the best four-letter words in the English lexicon. When people see it, their interest is immediately sparked. And it gets even better when that something free is tied into a contest. Not only is there a prize involved, but there’s also a win.   Every day,hundreds of businesses tap into the psychology of winning and getting something for free. And they do it through social media contests.   Take BMW, for example. The venerable German automaker put...