"The sites assemble these bits of data into brilliant databases and reuse the information to provide value-added service—but only within their sites. Once you enter your data into one of these services, you cannot easily use them on another site. Each site is a silo, walled off from the others. Yes, your site’s pages are on the web, but your data are not. You can access a web page about a list of people you have created in one site, but you cannot send that list, or items from it, to another site."