My two favorite social media platforms for business are Twitter and YouTube. As soon as it became available, we began using YouTube to help our clients create video content that was persuasive, personal and informative. First, it is a medium that makes it easy to absorb the content if is meaningful to its audience; second, it is a very good channel for search engine optimization. Plus, you can measure the number of impressions with one click.
Twitter is a great business intelligence and news monitoring tool, enabling one to quickly skim Tweets to spot trends, perceptions, interests and see quickly what the journalists I follow are reporting. I highly recommend that clients consider starting an account if for no other reason than to use it as a research tool.