I don't think FaceBook is worth $15.00 a month. I only pay that a year for LJ, Wordpress, LibraryThing, Flickr, and Dreamwidth and they are infinitely more useful to me.
I might be willing to pay $12.00 a year for FaceBook because I believe in supporting programs I find useful. I don't, however, feel I should be gouged. Charging more monthly than other, more useful services charge annually is gouging.
There's nothing about FaceBook that I would want to pay that much for. I will drop it without regret if they actually do charge that much monthly.
Annually, I might go for.