What if you want to move your photos from one website to another? How do you choose which social networking sites to commit to? What about when a Web 2.0 site dies? This happened with MP3.com and Stage6. Or what about if your account gets closed down? There are documented cases of people whose Google accounts were hacked so those accounts were subsequently shut down – they lost all their data.
These are examples of Metcalfe's law [link added] in action. What should really happen is that you keep all your data on your website and then aggregators can distribute it across the Web.