ONCE upon a time, the story goes, we lived in a broadcast society. In that dusty pre-Internet age, the tools for sharing information weren’t widely available. If you wanted to share your thoughts with the masses, you had to own a printing press or a chunk of the airwaves, or have access to someone who did. Controlling the flow of information was an elite class of editors, producers and media moguls who decided what people would see and hear about the world. They were the Gatekeepers.

Then came the Internet, which made it possible to communicate with millions of people at little or no cost. Suddenly anyone with an Internet connection could share ideas with the whole world. A new era of democratized news media dawned.

You may have heard that story before — maybe from the conservative blogger Glenn Reynolds (blogging is “technology undermining the gatekeepers”) or the progressive blogger Markos Moulitsas (his book is called “Crashing the Gate”). It’s a beautiful story about the revolutionary power of the medium, and as an early practitioner of online politics, I told it to describe what we did at MoveOn.org. But I’m increasingly convinced that we’ve got the ending wrong — perhaps dangerously wrong. There is a new group of gatekeepers in town, and this time, they’re not people, they’re code.

Today’s Internet giants — Google, Facebook, Yahoo and Microsoft — see the remarkable rise of available information as an opportunity. If they can provide services that sift though the data and supply us with the most personally relevant and appealing results, they’ll get the most users and the most ad views. As a result, they’re racing to offer personalized filters that show us the Internet that they think we want to see. These filters, in effect, control and limit the information that reaches our screens.