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.