When people, jaws dropped, heads tilted, ask me why I don't have Facebook, I usually reply something akin to "I've been on the internet too much." or some other vague answer. The real reason is more complex, and be summed up by main points
Security: knowledge is power. Know thy enemy. With no FB comes a reduced risk of someone finding potentially dangerous information. There are people who can find you regardless, but FB in essence brings stalking to the masses.
Friendships: My true friends contact me because they care about our relationship. FB creates false pretenses by allowing the extreme ease of communication. I.E. a true blue is someone who occasionally calls just for a good chat. Someone who writes on your wall gets a big question mark for how much they care about the relationship.
Top 8 drama: Not exactly Myspace, but FB drama through invites, rejections, and pretty much everything else possible. Not interested. Thanks.
I understand why most people use it, and I don't blame them too much; I just blame them for creating pressure on others to join. See Randy Marsh on South Park.