This is a great post by Guy Kawasaki about why he decided to create a FB Fan page for his new book launch, rather than a separate custom website. I think “speed of implementation” is a great motivation for going this route. However, I might argue his point that a website would cost significantly more than a custom souped-up Fanpage.
I have been aware of the “opt-in” content reveal possibilities for FB pages, and only my lack of time to mess around with FBML coding has prevented me from further experiments with it (that and the rumor that FBML is going out and everything will be implemented as iFrames soon… [sorry for the geeky aside…]).
Anyway, it’s very difficult to do real SALES on an FB page, so you will need something online that can handle sales and link to it from any FB pages you create. (For example, Guy includes links to Amazon, BN.com, Borders, and Indie Bound bookstores.) If what you are promoting is not being sold by a third-party, you’re still going to need a “sales page” outside the realm of Facebook. In those cases, I say – Do both.