Facebook’s new “Discover People” feature is a new way of doing an old thing
If you are somewhat surprised by Facebook getting away with this, consider this. First, it's only showing information that's publically available on profiles, and the choice of what to share is ultimately yours. Second, one can already see who's attending or interested in specific events by visiting the event's Facebook page. Same for looking up people by specific criteria. All Facebook seems to be doing is providing a more straightforward way of accomplishing things that are possible already.
The feature is coming to users on Android and iOS around the world as we speak.