I would have to agree about LinkedIn. All the social networks are good for businesses if you use them correctly. I find a lot of people use their pages for both social and business, which to me is wrong. I feel that you should have one page for your business and another for your social life. When I am looking for something to do with your business, I am not really interested in what "you did with your bf at the party the other night." That is why LinkedIn is the best, hands down.
I prefer a more professional than social network. Most sites are nothing but dating sites, such as MySpace(which I believe should be banned, due to the smut which exists on it). I also am a member of LinkedIn and enjoy it very much.
Which Social Networks are best for you depends on what your goals are:
LinkedIn for networking with decision makers, other consultants and agencies. There is a widely recommended book on how best to use LinkedIn called I'm on LinkedIn - Now What. If you went to a prestige school or worked for a major company you can get strong results there quickly.
Twitter is essential for everyone. You can often use it reach people who are so busy and difficult to contact other ways very quickly. Being able to add you to their Twitter lists lets those who want to collaborate with you keep up with and share your blog posts and latest promotions.
FriendFeed because you can feed your blog posts there automatically and subscribers there boost your blog subscriber counts at FeedBurner, FeedBlitz, etc. Excellent for discussing complex topics with those you contact at Twitter.
StumbleUpon because it drives more traffic than any other Social Media site (unless you hit the Buzz page somewhere) and sends residual traffic.
If you have a business that operates primarily locally, getting active at MerchantCircle and/or Yelp can be excellent. Review businesses there you frequent and recommend and those in your niche and it is likely they will review you. Reviews will be ever more important as more people make decisions by reading what others believe.
Every business that has a physical address should make sure they are listed in the major Local Search directories. There is a inexpensive, fast way to make that happen (as a first step).
Be sure to claim your Google Maps listing manually to ensure no one else does. I know a business that had their excellent reviews hijacked there by a competitor and that made a huge difference in their revenue until they got that resolved.
I highly recommend picking up a copy of Tamar Weinberg's book The New Community Rules. It is the best way to understand each Social Network, learn what not to do and how to use them.
If you are going to use Facebook I recommend you first get The Book I'm on Facebook - Now What. Using Facebook for business is not easy and many have had to start over completely. Without the book you probably won't be very effective.
If you have any questions I invite you to ask them. I have difficulty staying active in all the communities I am in so if I don't answer write about here the fastest way to reach me is by leaving a comment in my blog or at Twitter.
You can find me as GrowMap with the same Avatar in other communities and on Social Media sites across the Web. I recommend my Best of GrowMap for more information on proven strategies to grow any business or blog.