As you have probably seen by now, the new kid on the block for social network sites is Pinterest.
They obviously have the same problems as the others have had to deal with regarding people abusing copyrights and with their focus being on images, there are already people trying to abuse their system and use other people's images illegally.
Pinterest are in talks with one of the companies that Google use to identify copyright infringements on Youtube and will be dealing with abuses as they implement new systems.
In the meantime, they've also implemented a special "nopin" HTML meta tag that lets website owners prevent "pinning" on their sites. Flickr have already implemented the tag, preventing "pinning" for copyrighted and protected photos on their site. Previously Flickr was the third most popular source of pins on Pinterest, so this update will likely have a big impact on both sites.
If you want to protect your own images you just need to add the following code to your <head> section of your web pages.
<meta name="pinterest" content="nopin" />