I have found several businesses that use their Facebook account exclusively, and do not have a regular website. If it’s free for businesses like it is for regular people, it would be a pretty good reason to use Facebook for their business. I don’t know if Facebook charges businesses, but if they do, it makes sense to only have one (a website or a Facebook account).
Are you talking about the Facebook logo on your toolbar? If you’re already logged in to Facebook it will just open it up. If you’re logged out of Facebook then you’ll have to log in.
When I click on it it simply sends me to my Facebook because I am already logged in.