"If you don't want people to share your photo, don't put it on the internet."
-vast numbers of people on the internet, 1995-2013
This refrain is among the most common threads in the great internet copyright wars. It's a facile argument for its ignorance of 3rd party uploads and of the blind resharing of photos tagged "attribution required" at upload, but I digress. Correct or not, the fact that internet content is so often copied without credit carries its own chilling effect on what gets posted.
With too many people unwilling to credit or pay photographers for their services, many photographers actually do follow the above advice. Some post nothing at all, or only upload small samples of their full portfolios.
The relative secrecy of these visual artists is a shame. Some of the unshared work I've seen in person is aesthetically spectacular, or scientifically informative, or both. And the losers in this situation are us, the users of the internet, because we will never see the amazing work not shared.