considering that for a long time facebook didn't have "usernames" and everyone was identified by a number this doesn't seem strange (especially as my faulty memory might mean that they created usernames post 2005)
I mostly use that to see public but normally inaccessible pictures of people I am not friends with (i.e. pictures they've been tagged in).
Try it yourself (for some reason this only works on desktop mode): https://www.facebook.com/search/{$PUT_THAT_ID_HERE}/photos-o...
I never had any illusions that Facebook wasn't spying on me: I just had illusions that they wouldn't be so stupid as to sell the only thing that makes them valuable in a raw format to customers.
Google at least just sells you targets.