> Looking at Flickr, it looks like the CC-licensed collection is about 100x larger than the PD-licensed collection

That's likely because CC-licensing has been available on Flickr since 2008 [0] and they only recently started offering the Public Domain license as an option in 2015 [1].

It's good to see them finally offer more license types but they are so late to the game that many people like me just self-published or posted to Wikimedia Commons under the PD license instead, for all those years. I'm sure my images get fewer views than they would on Flickr but I can't be bothered to switch now.

[0] https://www.flickr.com/commons [1] http://blog.flickr.net/en/2015/03/30/flickr-now-offers-publi...