Jeff Atwood has posted a entry Avoiding Walled Gardens on the Internet that advocates using open internet tools and avoiding products such as LinkedIn or Facebook. The reason we should avoid these tools is to avoid locking up our contributions in other people's safes. Since LinkedIn and Facebook control their sites your contributions to their sites are locked up forever inside their infrastructure.
Jeff's opinion has a ring of truth to it. However, I don't think this means you shouldn't participate in the closed networks, just be certain that if you contribute to a Facebook that you can also use your contribution in the wild and wooly web.