Hard to say what data they still retain and for how long. However, the sooner you delete your FB account, the less data FB is likely to have on you in 5 or 10 years, assuming data retention periods, etc.

I found the process easy to complete using the directions in the link. It was quite liberating to have my account deleted, actually. Don't miss it at all, although it left some friends and my mom scratching their heads.