You'll notice that Facebook, Twitter, and blogging are the top three on almost all fronts. This includes use right now and expected use in the next one to three years. The reason for this is pretty self-explanatory--they are most likely to bring in actual clients.
The best data that we have specifically on law firm use shows the same three platforms leading the pack: Facebook, Twitter, and blogging. However, legal blogging (or legal content writing) is the most widely used.
This isn't surprising because while social media sites like Facebook and Twitter are important for attorneys, they are more critical business that sell goods or non-professional services. In other words, it is obvious why Pizza Hut might want to spread messages about a new menu item or promotional offer to everyone on Facebook. Almost everyone eats pizza and seeing a Facebook message about a new specialty option may be just the push someone needs to make a call for dinner.
It doesn't work in exactly the same way for law firm. Sure, any sort of online exposure is probably a good thing. But more comprehensive tools--like a blog--allow an attorney to project experience and competence in a field in ways that a very brief Facebook message or tweet cannot. It is best to view Facebook, Twitter, and other social media sites as complementary tools for a law firm, with law firm blog writing as the centerpiece.