Don't forget their "interestingness" algorithm that would determine which photos get to the top position in groups and other shares views. Way before ML ranking.
Funnily enough, I think it was also one of the early warnings of what's coming. The problem with the Flickr algorithm was that it ended up promoting a very predictable style of photos - mostly HDR landscapes - which in turn encouraged more photographers to emulate it. In its early years, it was a fantastic place to learn photography. In the "final" years, it was extremely homogenous and, frankly, boring.