Facebook reported an incident where some employee systems were compromised by a "sophisticated" malware attack. They have strongly stated that no user data was compromised in the attack, and that the penetration was quickly contained.

Because of this quick containment and reduced impact, it appears as though this is only a Level Two attack. It is being investigated by Law Enforcement, and it does appear to be intentionally malicious, so more information may come to light which might affect this ranking.

• Facebook Security Blog: Protecting People On Facebook
• Ars Technica: Facebook computers compromised by zero-day Java exploit

Update

On 2013-02-19, Apple announced that it had also been penetrated by this attack. Together, these two attacks form a Level Three attack.

• Gibson Index: 2013-02-19: Apple Computers Hacked (Level Three Attack)