Excerpt from this article:
Using a link to Facebook posts, Five analyzes the language in which we write, and determines our relative affiliation to five personality attributes: openness, extraversion, agreeableness, conscientiousness and neuroticism. It then shows comparisons with famous people (based on their public writings and statements), as well as your Facebook friends.
Based on the initial responses to the site on Twitter, “people seem to identify pretty strongly with the personalities we generate,” said Nikita Bier, the co-founder of Five, which is working on a product for online conversations that will use similar technology. “Only about 10 percent said we were outright wrong about them.”