After reading Kirsten's post on the DDK Portraits blog, the one thing I can determine is the simple rule of the Internet: If you don't want people "sharing" your stuff online, don't post it online.
There is no possible way to enforce your copyright as a photographer online, unless you want to sue every person that shares your stuff or you want to take the time to send out takedown notices to hosting providers. Both are very arduous processes and consume massive amounts of both time and money.
The bottom line is that if you want to post your stuff online, be realistic about what people are going to do with it. You can't honestly post to the modern-day social web without thinking that, at some point, someone is going to discover your work and share it. Traditional copyrights are simply not compatible with the social web.