For quite some time, advertisers could scrape Facebook user IDs for those who shared, liked or otherwise engaged on anyone’s Facebook page or account.  This means you could scrape the IDs of those Facebook users who engaged or shared with competitor’s Facebook pages and then you could target ad campaigns specifically to those exact people.

The process was technically against Facebook’s terms of service, however there were many tools that people could use to scrape IDs for ad targeting with little to no consequences, although Facebook would disable or throttle many of those third party tools that scraped IDs.  However, Facebook has changed their ad serving settings so that you can no longer target users simply by scraping their Facebook IDs.

If you are advertising a Facebook app however, you can still target by user IDs.  However, that change means you can only target those who download or use your Facebook apps, and you must include the Facebook App ID for cross-referencing.