Over half of the people using it, use it daily.Edit
Think of how many subscribers or monthly visitors you have. How many of them, honestly, visit you daily? What if you could increase that number simply by having a presence in a free tool they use daily already.
The opportunity exists in Facebook. Of the 35 million people currently using Facebook, half of them are using it daily.  Why do they do this is the key observation here.
There's no way I can speak for 17 million people, I can only tell you what I see. And what I see is that people who log in daily connect more with the people and information around them. Why can't that be you, and your information?
Your clients -- and your competition's clients -- may already be on Facebook.Edit
With 35 million people registered, and new signers showing up so fast, someone from your desired online demographic is on Facebook.
Why is this different, this time? What does it matter that your clients use Facebook already?
Because with Facebook comes a completely different way for you to build trusting relationships with your clients and prospects. Since they must join your group, or add you as a friend to interact with you, it's completely opt-in.
You'll be able to have deeper interactions with them, through photos, videos, private messages, public messages, or through custom applications you can build or use to enhance their experience with your company.
Facebook friends are willing to continue the conversation.Edit
Facebook is for social information sharing and interaction. It make sense that people who are already in the mode of action would continue interaction with you or even follow you to your site.
The traffic to your site isn't, by far, the most important benefit of Facebook for business. It's worth noting, but the real opportunities with Facebook are to get better information, faster, and to have better social connections to both existing and new contacts, be they client, friend, acquaintance, or colleague.