Users can request an alert when their account is accessed from a device they haven't used before. And when Facebook notices unusual activity, it requires users to answer some questions before the second login goes through.
When Available: Some changes are already available, while others will come in the next few weeks. Not all users will get them all at the same time.
The Backdrop: Facebook has been facing criticism about how it handles users' privacy. The security improvements show Facebook's efforts to maintain users' trust in the way it handles private data.