As of today, those Facebook advertisers across the world that leverage Custom Audiences will have access to new Facebook Lookalike targeting. Lookalike Audiences allow advertisers to expand upon specific user sets that have been uploaded to find a larger (but still targeted) audience.
Custom Audiences allow advertisers to upload a list of email addresses into Facebook for specific ad targeting to the given set of users. The Lookalike audience functionality leverages algorithms to detect other Facebook users that match a given set uploaded to Custom Audiences. Essentially, Lookalike Audiences lets users amplify their uploaded lists by finding like-minded users.
As of yesterday, this new feature is out for all Facebook advertisers and can be found in the Facebook Power Editor tool. For more information see the official blog post.