Private blog network is a blog network such as tumblr.com, livejournal.com where people can create free blogs. Free blogs are not as powerful compared to self hosted blogs. The private blog network is usually consists of expired domains that was used before. The reason is that expired domain is much easier to rank. Many webmasters decided to buy domains and set them up as blog and use them to distribute content which include links to their other websites. The advantage of setting up your own private blog network is because you can control the content.