How do they use it? Does it have a good programmatic access? Because UX-side, I'm surprised they still exist. I don't know of a single photo-related web site that has worse UI and is more annoying to use than Flickr.
This is extremely biased. A lot of people, myself included, find it easy and pleasant to use. Honestly, I'm not aware of a single alternative with a better UI/UX -- 500px maybe?
As for programmatic access -- Flickr has a good API interface.
SmugMug is great! It's a little bit different from Flickr though -- SmugMug is more of a personal photo hosting/portfolio website, while Flickr is more about photographers community. I believe the social aspects of Flickr are much more important than it's actual photo storage capabilities!
My trouble with the UX is that it is slow, slow, slow. I live in Frontier country and I have DSL, but there are many photo web sites that are faster than flickr.
I hardly upload anything to flickr anymore because the interface for that is so slow.
As for programmatic access -- Flickr has a good API interface.