I think the major point is being missed here. Ultimately this discussion all boils down to demographics. The professions are ageing. Their client bases are ageing - both mainly populated by 50 somethings.
Our future clients are going to come from the generation that are 15 - 30 now, and these youngsters literally live their social lives on Facebook. It's the online environment they are familiar with and the communication medium they find most comfortable and convenient. When was the last time you received an email from a teenager? Exactly, they think email is for the government.
So, I argue that we need to think like McDonalds. They know where their future customers are and they spend a lot of time and money capturing their hearts with "Happy Meals" and free parties. The burger behemoth reaches out to its future customers now, on their terms.
Facebook is the Happy Meal, Twitter, LinkedIn and email are for grown ups, and what teenager wants to hang around with grown ups?