Advertisers in the beta have seen lower costs per action than with traditional targeting options. Lookalike Audiences can be created after an advertiser has uploaded a list of first-party data, such as customer email addresses, phone numbers or user IDs to make a Custom Audience. Facebook’s algorithms analyze the Custom Audience and produce another audience segment that is likely to have a similar customer profile. The advertiser can then create any Facebook ad type and target it to the Lookalike Audience. No personally identifiable information is shared back with advertisers and Lookalike Audiences can only be used within Facebook, not exported for email marketing or other ad targeting.
Facebook ‘Lookalike Audiences’ help advertisers reach users similar to current customers, others in their database
Tools like this can turn Facebooks advertising offer into a cash cow similar to Googles AdSense. Last not least because Facebook, same as with Google and AdWords and AdSense, is the only current provider for something like that on a big enough scale.
And you need scale for a matching of audiences to be attractive to advertisers.
So, this could be huge.