Too many business people currently using, and encouraging the use of, Social Media look like they are standing on a street corner with a bull horn. They are missing the point: Social media is a two-way conversation. You have to be social, for social media to work.
Facebook in particular is a SOCIAL venue. Think of it as a party, or better yet, the office water cooler. How long would you stand around talking with someone who 1) only discussed himself or herself, and 2) refused to carry on a conversation with anyone else? Not long!
LinkedIn, on the other hand, actually asks you to tell visitors what you are working on via status updates. Twitter is a mixed bag giving you access to current and potential clients, and the ability to monitor mentions of your brand.
Bottom line: if you want to market yourself and your business spend time on LinkedIn. If you want to connect with family and friends, socialize on Facebook. And if you want to know what people are saying about your business do a Twitter search.