Twitter is a popular choice among lawyers and other business owners, which allows for quick "micro-blog" posts that communicate basic ideas. Its usefulness is not limited to business- everyone from satirical figures (Obama's Teleprompter, anyone?) to celebrities.
Why use Twitter to promote your business? Well, people can follow your Twitter feed and will be exposed to your business each and every time you "tweet." If you can provide them with useful information, they will be likely to click on links in your posts, along with keeping you fresh in their mind. I recommend the use of Twitter for lawyers, because it does not require a lot of upkeep and it can help you improve your blogging skills if you should ever want to start a blog later on.