It was 2005. The web was a different place back then. Although it's a bit earlier, ICQ used monotonically increasing user IDs as its addressing mechanism.

Facebook has restricted apps to only receiving app-scoped IDs since 2014.

Only took them 10 years?

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)

One can still find the id, even when a person has chosen a username that masks it. Right click the block option of a user you are not friends with, copy the link and voila, you've got the user id now.

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 know they exposed identifiers in friends lists as well, long into 2013 as I recall. I remember hanging out with friends and grabbing the Inspect Source HTML from their little "Friends tile" that had a ranked list of Identifiers that were based off of how much their algorithm thought we were friends. It would display a random selection from the top 50 or so but all 50 were in there and were fairly accurate as to who I interacted with most on Facebook. My friends and I would log them all then compare notes.

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.

That deserves a serious wat. How could you be so dumb as to release internal non-opaque IDs.

I believe we called that, "2003 engineering decisions."