Yes you can both deactivate (soft delete) an account and you can actually say to facebook to actually delete it. Deleting an account does not allegedly remove your data from the Facebook servers. Deleting items however will remove your data from the servers. The idea is that you should delete each little thing first, and then delete the actual account.
From the FAQ at http://suicidemachine.org/ :
"Facebook and Co. are going to hold all your informations and pictures on their servers forever! We still hope that by removing your contact details and friend connections one-by-one, your data is being cached out from their backup servers. This can happen after days, weeks, months or even years. So merely deactivating the account is just not enough!"