What is this? Ask a user a series of questions and define their personality based on the answers.
Why should I do this? People love to be defined. Some wear it as a badge of honor, others are surprised by it, others vehemently disagree - but in all cases this is an opportunity for the user to share something on social media to compare with their friends. A mechanism whereby you help a user define themselves in either a serious way or a humorous way, is an engine for social sharing.
Who else is doing this? “Which ____ are you?” is a style of content that’s older than the internet. The web just makes this more interactive. Buzzfeed puts out some particularly obscure ones which get a high amount of social sharing, such as Which Muppet Are You? and Which Breakfast Cereal Are You?
How can I implement this? Picking the subject is a challenge, but a good base template is the aforementioned “Which ____ are you?”. From there you will need to decide what has the right combination of relevance to your target audience plus quirkiness / humor potential to maximize the resultant social sharing. Creating the horoscope-style quiz itself is a fairly simple exercise - a number of plugins for jQuery exist for this purpose, such as SlickQuiz.