In terms of being a cloud service for photo storage, they can't compete with Google Photos, iCloud and other mobile services that have the benefit of being more deeply integrated and available as a default on their respective platforms.
The social network aspect is obliterated by Instagram, Snapchat and the like.
The photo reselling part I'm not familiar with, but it seems to be lacking in a similar fashion.
In short - their high point was the time when they primarily catered to photographers. All three aspects of the service were useful for that crowd and perhaps if they kept that position and developed advanced features that appeal to that particular crowd, they could have remained strong. But once they started reaching out to the general public, they alienated their original audience and at the same time failed to compete with (originally) smaller services that provided a better experience in each specific field.
That's the way I see it.
They want about $40USD per picture of 72dpi, which is waaaaaaaaay overpriced. On Shutterstock, you can get 2 images for $29 with the most expensive plan (not sure about the resolution, though).
No wonder they failed.