I was slow to adopt use of Facebook, whether as a business tool or as a personal tool. However, I have changed my tune and now see the usefulness of using Facebook for promotion.
The Fan Page is separate from my personal page. Good. My initial worry was that friends and family would get inundated with book promo postings, but this hasn't been the case.
(You can open the page without being signed in to Facebook, and can do so even if you do not have a Facebook account. Facebook gives you a free vanilty URL once you get 100 fans).
There are, of course, many benefits to having a "Fan Page." It's a fun way to reach an audience you may not otherwise have tapped into. You can get demographics and stats about who your fans are (just general demographics - no stats are linked to individuals by name). It's also handy for planning events such as readings & signings.
I have no idea if Facebook and Fan Pages lead to sales in the end, but the page is certainly great for at least getting your name out there.