Facebook knows a lot about you, most of which is shared with friends. Much of the information stored and shared on the site could be used maliciously, and so it is important to ensure that you are careful about your data online. As such, it is worth being aware of all the data that you have given over. You might not want to delete it all, rather it is imperative for you to know what exactly you are sharing.
Facebook collects everything that it thinks might be of interest to you and uses that to target ads at you. It does that through two main routes: encouraging you to like things so that they are on your profile, and then guessing other things based on your likes and other activity. That information could be private and you may not want people to know what you are into. In this case is easier to remove your interests or give Facebook entirely different ones.
Birthdays on Facebook are traditional: everyone posts on your wall, and soon after that you have to send out a message thanking them for their wishes. But they are also potentially damaging – your birthday is actually very sensitive information that can give people more easy access to your bank and other personal accounts.
Your home address
You probably would not give out your home address to a stranger. But you might be doing that accidentally while you use the site. People often list their address while planning events, or even on their own page. If you need to tell people where you are, it is best to do it privately rather than posting online for the whole world to see.
Your school or workplace
This can be a nice way of catching up with former colleagues or school friends, and even getting jobs. But it can also be a terrifying way for unwanted people tracking you down. If you want to keep the information on there but would rather not show up, you could change the name slightly, so that it will be obvious to anyone who visits but people will less easily be able to find you.
Old, embarrassing information
This is not so much of a security or privacy concern – though it might be – as it is about conserving your pride. Because Facebook is now so old, it has changed a lot – and, probably, so have you. That could mean that there is still some embarrassing information in undiscovered and unwatched parts of the site. It is worth checking and making sure that there is no odd or shameful bits of information that you would rather be rid of.