I'd just go with something like "Sorry, I won't be joining that one but good luck!" No need to explain. (Or even respond at all, unless it was a personal message that asked you specifically. I'm assuming a mass mailing here?)
I think that's the best option, unless it really was a big mass mailing, in which case you're free to ignore it.
It also may depend on how you use Facebook. I use it socially. I don't become a fan of my friends' businesses when they send requests, because I don't use Facebook to advertise. I don't join millions of groups, because frankly, I don't care (I did join the ehell page, though
). I don't use it to raise awareness for my pet causes, because I figure if my friends don't already agree with me, a Facebook app isn't going to change their minds (and really, as an earlier poster said, aren't we all aware of breast cancer? It's not some big secret). I use it to keep up on my friends' lives, and take the occasional stupid quiz, and that's it. If someone asks me why I didn't become a fan of their uncle's sister-in-law's friend's new restaurant, it makes it very easy to just say "sorry, I don't use Facebook that way."