MySpace and Influence
With all the focus around open standards, open networks and all things user-generated, the value of the 'traditional' closed network tends to be minimalized. The distinction between a MySpace/Linkedin profile and a blog is a great comparison. I'm (almost) free to do whatever I want on my blog. But frankly the lack of structure to a blog, which is greatly valued by some, is most likely a deterrent to the mass market. For those folks, the value that most closed networks bring are structure and an interface that's more easily adopted.
MySpace/Linkedin are great alternatives/complements to the typical blog. I don't need to write periodically to my profile, just enough one time to capture my shared interests, whether business or personal. The closed network then generates 'demand' for my profile by exposing the tags/descriptions via structured search (more user-friendly than an open search on a search engine). So long as the network is large enough or growing (which comes with mass market adoption), exposure of my profile is sustained regardless of new content. In comparison, demand on my blog correlates very closely to the sheer amount of content I put on it. Without new content, a blog's value to search engines decreases, reducing visibility. Unless you actually enjoy writing, a blog is a fair amount of effort over and above the network profile. Additionally, blogs have few if any of the structured search/marketing tools that closed networks offer.
Beyond the social/dating aspect of MySpace and other social networks, the sheer empowerment of the individual through a social network profile and 'friends' network is fairly awe inspiring. (Potentially even more so than the influence you can have through a blog) Consider comedian Dane Cook and his 1.4MM friends. These are MySpace users who have self-selected to receive blog 'updates' from one of their favorite entertainers. Should Dane head to any city, he has a 1.4MM marketing list to work with. Name me an individual blog with that reach... I don't think it exists.
Update: Thanks for reminding me Jeffrey. Here's my Myspace profile - RoChoy