On Tuesday, Facebook announced the launch of website and mobile app custom audiences to marketers. What this means is that Facebook’s custom audiences targeting will not only utilize people within Facebook itself, but those who have visited your site or your app.

The company explains in a blog post, “For instance, a bike retailer could reach people who started designing bikes on its website but didn’t make a purchase. Through custom audiences, the company can reach these people via desktop News Feed and encourage them to finish customizing online. Or, a mobile travel app can deliver ads to people who have downloaded their app but haven’t used it in a while and encourage them to book getaways within the app.”

And the lines between on Facebook and off of Facebook just got blurrier. Facebook has addressed the inevitable privacy concerns as well, however. The offering does not utilize any personal information about users. It works essentially just like the existing Custom Audiences feature. The company makes a list of people who should see specific marketers’ ads, but doesn’t show the list to other companies, and doesn’t use the info it gets through the program to build profiles of users’ interests and activity. They’re also providing an opt-out option.

Additionally, Facebook is testing changes to the “About this ad” feature that appears when a user closes out ads. It’s going to start giving users more info about why they are seeing specific ads and why they were targeted. More on the privacy efforts here.

For advertisers, the new targeting comes with all ad formats and other targeting capabilities, but it was really designed with mobile app developers in mind.

“Website custom audiences is similar to existing remarketing tools that allow marketers to reach people who previously interacted with their desktop websites. Many marketers already use third-party tools to remarket through Facebook Exchange (FBX),” Facebook says. “We anticipate that marketers will use FBX and website and mobile app custom audiences in different ways.”

Facebook does note that advertisers that have a lot of products and advertise to multiple audiences will be better off using FBX.

Initially, website and mobile app custom audiences will be available to a limited number of partners, then will roll out globally in the coming months. Facebook provides code that marketers can place on their sites or apps. Look for it when Facebook rolls this out more broadly.

Image: Facebook