The reason it feels like such a terrible deal here is that you aren't paying $3 CPM to reach a new audience, you are paying $3 CPM to reach your audience. Sure, Facebook can argue "it's the most targeted audience possible, they're Mavericks fans!" but the counter to that is "of course they are, it's my brand that put this audience together".
Brands use Facebook because it's far more user-friendly than trying to collect your fans together on your own website, and therefore it's a better solution - easier for your fans means more will do it, so you have more people to market to. However the comparison is that they're people who have chosen to subscribe to you, and therefore the pricing comparison is not to digital advertising, but to writing to fans on a website, or a blog, or through email newsletters. This is why it feels like, and arguably is, a rip off.
As to the argument that it's needed from a user's point of view and that Facebook happens to be able to monetise to help their users (e.g. jeffwidman's comment: http://news.ycombinator.com/item?id=4785852) I'm not really sure. It's probably a valid point. I'm not a Facebook user myself (but my marketing work occasionally covers Facebook), my personal feeling is that if I follow a brand it's because I want to see their updates. All of their updates. In my head, it's the same as signing up to a newsletter, or subscribing to an RSS feed, just through a convenient third party. But as I don't do it myself, I'm happy to consider myself wrong, or in the minority, maybe if I was an active user following many brands that I care about I would find that getting 100% of their messages would ruin my Facebook experience.