I was working with some friends last night who happen to own an eco-tourism lodge and of course with the economy that business is in a slump. We looked into different forms of internet advertising last year with dismal results. So this year, we turned to Facebook, since it has over 400 million users, it covers a huge potential market.
The advantages stood out immediately:
Ease of set up.
Pinpoint demographic targeting. You can target so accurately that you can actually reach only ONE specific person if you know their info. If you want to target single women turning 30 on July 10 who live in a particular town and are interested in books, knitting and bungee jumping... you can do so.
Choice of CPC or CPM.
Ability to set your bid per click and daily budget.
The disadvantage of this for a small business is the price. The bids per click are in the $1.00 range. The minimum daily budget you can set up is $1.00 a day, $50.00 per day is the recommended starting level. *gulp* I know it's worth it for the demographic accuracy that Facebook provides, but still....

The free alternatives, still using Facebook are to set up an Fan page or Event page.