I think this is it. Flickr content is much more searchable via tags, Facebook not so much. The focus in Flickr is subject matter and actual photography, with Facebook it's more about people and experiences within networks. Many Facebook photos are uninteresting to people other than the friends of the originator.

Publishers regularly search Flickr for photos to license and use. I'm a point-and-click shooter, but out of the blue I received two messages asking to publish one of my photos (which I had tagged descriptively). I was paid, and I learned how to complete invoices and W-9s, and had the joy of seeing my photo published with credit in a food magazine no one reads (Whole Foods sold it briefly). Yahoo could better highlight Flickr's ability to connect publishers with casual photo-takers, by facilitating thorough tagging of photo contents and improving the search ability. Flickr's definitely a much larger and open body of content than Facebook for this purpose.

There's (at least) one more level of granularity between professional and amateur though -- prosumer. Camera gear <$4000 && >$1000; a serious hobbiest if you will. I think the GP was saying that Flickr should market harder to that vertical, which makes sense to me because it's so much larger than the pros.

I consider myself a semi-serious hobbiest, my rig fits into the conditions above, and I still pay for my Pro account. I post a link to the album on Facebook, though.