I joined fb personally (not for the b&b) over a year ago after my sister and cousins nagged me to. People say it's about staying connected, but in my experience, it's about broadcasting every mundane aspect of your life to people you can't be bothered to stay connected with in the real world.
Anyhoo, regarding using fb for business... a friend of mine and his wife own a baby-apparel business. It's a small business, a cottage industry, but they appeal strongly to a small niche market. Their products are in just a few stores in a few cities; and they sell at shows and markets.
They started a fb group for their business and use it for outreach to the group members. They send invites to the shows/markets they'll be selling, send news about new stores they've picked up for distribution, new product designs. Of course this info only goes out to the people who have already joined their group. So it's not likely reaching new customers, but it could be a good way to keep in touch with an existing customer base and encourage repeat sales.
There is definitely some overlap with a blog in terms of functionality. The invites and news items via the fb group are the same kind of thing that a business would put in a blog. The difference is that readers must come to a blog to get the info (or subscribe to a feed), whereas the fb group broadcasts go to the customer. And it reaches the customer in a place they have already chosen to spend a LOT of time (many of them anyway--it's not called 'crack book' for nothing).
I think a fb group could work for a B&B, but more for boosting repeat business and referrals, not for plucking new guests out of thin air..
