A person with exceptional skill in finding information with computerized search.
A person with exception skill in searching through social networking sites to gather information about another person.
When used pejoratively, 'data hound' may refer to exceptionally obsessive people whose only interest in Facebook is forensic, or who are capable of deducing someone's entire dating history by looking at status updates.
Steve's such a data hound that it only takes him fifty-two seconds, on average, to reply to a question on Yahoo! Answers!
Holy shit, that fucking data hound figured out that I used to date Steve in college... just by looking through every photograph of every friend I've ever had on Facebook.