A flaw in Facebook's security could come from any of many directions, but most of those are out of your hands. Session hijacking via a Trojan horse on your machine, simple phishing, or password theft via a Trojan would also compromise your account. Finally, sharing passwords with any other sites means possibly giving those sites the keys to your account. Good virus protection catches 2/4, the third (phishing) is a matter of making sure you never enter your password into a site you haven't confirmed to be "facebook.com" (I'd suggest setting up automatic login via another service, for example, a Google account, thus avoiding typing in your password at all) Finally, not sharing passwords between sites is just common sense.
In general, keep your wits about you, don't hand someone access to your account, and you're as safe as the safest facebook user can be.