I am asked this question on a regular basis. People want to know which social networking site will help them market their business. While I wish I could give you one site that can handle all of your marketing needs I can't. Each social networking site has its pros and cons when it comes to marketing. You have to look at what each site offers and what you are needing in terms of marketing.
First we will look at Twitter. I would recommend everyone have a Twitter site if you are trying to market your business online. Twitter allows you to send short 140 character messages. You can develop a following of thousands on Twitter. It is a great free way to market your business as long as you follow some basic rules. Don't spam your followers. They will see this right away and delete you from their list. Offer valuable tips and tricks that relate to your field. It is okay to throw in an occasional message about your business. Use re-tweets. They show your followers you think what they have to say is worth saying again.
Next we will look at Facebook. It is harder to market your business with a regular Facebook page. Facebook is more of a personal place for friends and family, unless you are a well established brand or company. You can however create a fan page for your business. It may take some time to get a following of fans especially when your business is not well known. Be patient and provide valuable content, the fans will come soon enough.
Next we have YouTube. While you may be wondering how YouTube could help you market your business you would be surprised at how popular videos have become. Create a video about your business. Show how to do something related to your line of work. Videos are a great marketing tool.
There are plenty of other social networking sites out there. You have to look at each one and see what they have to offer. Start out with one or two and then go from there. Make sure to participate fully on whatever site you choose. Don't just spam message after message.