
Here are some fun facts about Facebook. According to their own Press Room page (which is brimming with delicious statistics), Facebook has 400 million unique users, 50% of whom log in at least once a day. More than 35 million users update their statuses each day. More than 3 BILLION photos are uploaded to the site each month. More than 1.5 million local businesses have active pages and more than 20 million people become fans of these pages each DAY. This is a very engaged audience, so chances are if you target your ads appropriately, you'll get a good response. As such, I think Facebook makes sense for small local and niche advertisers such as wedding photographers or local retail stores.
I'm still on the fence about B2B marketing on Facebook. It may not be the best outlet for that - at least not yet.
Here are a few more stats about user engagement. The average user:
Facebook's online advertising FAQ and advertising guidelines contain all the information you need about launching a campaign, but I've pulled out the bottom line info here. Read the Facebook Advertising FAQ if you need more details.
Audience Reach
Ad Targeting
Ad Specs
Here is a sample ad:

Payment Details
Here's a link to Facebook's Advertising Guidelines. Nothing surprising here. In summary: