Social networks such as Facebook are changing. They are becoming open platforms on the web, where commercial and other interests congregate to tap into the millions of users already there – and their treasure trove of personal data.

Third-party developers can add applications that tap into your private information, using it to fuel their own services, from “social” games to dating. Corporate marketers are also starting to swarm around the networks, looking for new ways to learn about and influence their customers.