Law.com's Legal Technology section has a great article by C.C. Holland to get you thinking about the ethical limitations on a lawyer's involvement in social media: Mind the Ethics of Online Networking. The article focuses mostly on Facebook, MySpace and LinkedIn and lawyers use of these social media sites.
As a lawyer who uses these tools and other social media tools, I am always cognizant of not soliciting for business and not directly answer legal questions. As the article points out, the various state bars do not have any bright-line rules on the use of these tools. To further muddy the waters, each state has its own rules on advertising and solicitation.
I use Facebook and LinkedIn as part of my professional brand. They are places where you can find more information about me. Hopefully, the information you find there and find on my blogs will give you a better understanding of my professional background and me as a person.
There are some basic principles that I adhere to and that other attorneys need to adhere to:
Be truthful. False or misleading communication about a lawyer or the services a lawyer provides are always going to be an ethical violation.
Put up a disclaimer. At the bottom of my page there is a notice that the blog may be considered advertising and that I am not rendering legal advice.
Be careful of direct communication. Posting on a blog is just making information available. Directly communicating with someone opens the door to the creation of an attorney-client relationship, even if you did not mean to create one. I posted a few months ago about the Massachusetts position on putting emails on a website: [Problem with Email on Law Firm Websites], [Problem with Email on Law Firm Websites] and [Problem with Email on Law Firm Websites].
None of this is a reason not use social media tools. If you are an attorney, you just need to remember that you are an attorney when using them.