Good article. Twitter is an excellent (and entertaining) resource. It is not, however, the end-all be-all of social media and/or online marketing. Lawyers should not expect to acquire significant numbers of clients over Twitter (or any other social media network) without becoming a part of and contributing to the community.
Your involvement on social media sites is somewhat similar to the newsletters and legal updates that your firm sends out. You get your name in the minds of your audience by giving them relevant and informative updates and information. Then, when they have a question, they may come to you to have it resolved.
Twitter, Facebook, LinkedIn, and other social media outlets are not like paid advertising. This type of marketing requires you to contribute (beyond just paying for a spot).