Facebook account deactivation is not the same as account deletion. In the case of the former, all your private information is still in the database and is still available to third party apps providers and advertisers as well.
Before you delete your Facebook account, you may want to download a copy of your info from Facebook. If you want to leave Facebook forever and have you account deleted, the crucial step is to not use your Facebook account for 14 days after you’ve requested deletion. The slightest interaction with Facebook and the deletion is canceled. That includes NOT logging in to Facebook using any client (app.), and NOT clicking embedded Like-buttons on other blogs or websites, and NOT logging in to other services using Facebook Connect (like Digg) etc.
How to Delete Your Facebook Account (as at June 2012)
If you are wondering what prompted this post, then know that several bloggers I know have decided to leave Facebook and they told me about their intentions. That gave rise to me checking out the differences between account deactivation and account deletion.