Social Networks carry a lot of private information about people. This can be advantageous as well as disastrous to individuals.
Giles Hogben of ENISA has been doing a lot of research work on security issues surrounding social networks. A position paper from Giles titled "Security issues in the future of social networking", has been presented at the W3C workshop on the Future of Social Networks.
Interesting bit from Giles's paper:
The biggest repository of personal images on the internet is not Flickr but Facebook (already with a staggering 30 billion images, while 14 million new images are uploaded every day). The largest number of personal profiles on the planet is held not in a government identity registry (at least not one we know about...) or one of the much heralded Federated Identity Providers but in the data warehouses of the Social Networking providers.