Back at the end of January, Facebook added secure browsing (https) to the list of security features and you should make the change now.
Up until the addition of https your Facebook credentials were passed as clear text to Facebook. This is especially troublesome if you are surfing on an open wireless network at say a coffee shop, because it allows people sniffing the wifi traffic access to your Facebook credentials.
Changing Facebook to use https adds a layer of encryption to your web communications that makes it much harder to crack your account. Luckily the changes are easy (though in normal Facebook fashion buried in menus) and will be largely transparent to the user.
From your home screen Click:
Account
Account Settings
From the Main Settings Tab, Click Change on Account Security
Place a check in the box under Secure Browsing (https), Browse Facebook on a secure connection (https) whenever possible
Click Save