Facebook can be an enormous drain on your attention, but it can also be used as a marketing tool for both individuals and your firm. Banning Facebook altogether will create unneeded tension between management and attorneys, and is unlikely to create a boom in productivity.
Lawyers are busy, or at least they should be. If Facebook is causing any employee to miss deadlines, not complete their work, or making them unproductive, the problem is with the employee, not Facebook. Any lawyer who places a priority on status updates over clients is not doing their job. Banning Facebook will just force an unproductive person to find another means of distracting themselves.
For lawyers that actually work, and are quite busy, granting them the freedom to spend five-ten minutes on Facebook seems worth it. Most big firm attorneys work long hours, and have limited social lives, letting them interact on Facebook will probably make them a bit happier, and happy employees are more productive.