Whilst I don’t have any intention of deleting my Facebook account, if I decided I’d like to delete my account then I should be able to delete it quickly along with all the data.
The problem with on Facebook it requires users to do the hard work. That is because, they encourage users to “deactivate” their account and the only way to be certain that your account along with all the data is removed from Facebook is to delete every single picture, comment and personal information from your profile and save the changes, and then email their customer service (who unlike Twitter actually get back to you) and ask for your account to be removed – then just in case they have disabled it, wait a couple of days and try to login, if a message pops up offering to reactive your account then you should email them again 
– if the message doesn’t pop up your Facebook account has been successfully removed.
However, I believe that the way Facebook are approaching user deletion is wrong, they should be able to delete effortlessly.
Update: Facebook have since changed the way the delete your account.
Let me explain from a generic perspective.
I believe that any company requires clear cut signals to tell them if they’re going in the right direction. Which is why I believe a company along with trying to generate revenue from its users (or even trying to get them to pay for services/features) needs to get the things which users don’t like about their product or service so they can learn faster.
Which means that the company is going to acquire more data and we all know what data means value. The value in this sense is to the company because, these failures (which is essentially what they are since users don’t like X or Y about their service) can teach them something about the market and can be corrected by the company either by terminating the particular feature, improving it etc.
This then creates a continuous loop as there is more data which facilitates the company to learn more about the market so they can make better decisions which of course leads to a better product or service. As a result the option to allow quitting on your service/product is important as it collects data and if you don’t allow quitting on your product or service then you are only going to be collecting positive data and ignoring negative data or to put a value on it 50% of the data.
Remember by collecting only half of the data then you’ll only be able to paint half the picture. Personally I’d rather be able to paint the full picture so I can assess the situation and ensure my company is heading in the right direction which is why allowing effortless quitting of your service is vital as you can collect data which facilitates you to meet the needs of the market and allows you to develop products which the market require which generates more revenue in the long-run.