In the letter, Zuckerberg explains that the team is doing away with the "networks" grouping for their now 350 million users. They say the adjustment is designed to help users choose who can see the content they post.
The team will help users adjust their new privacy settings by making recommendations based on current privacy levels. That's where Facebook can really take control of how the content is shared.
As it is now, most people choose not to share their information and photos with "everyone." By choosing to share within a "network," say, a work or regional group, users could have a sense of a smaller community of people seeing what they post. That's at least a safety net and a way to feel like your information is actually somewhat private. But, as TechCrunch points out, if Facebook decides they want to go back to a more Twitter-like presentation and encourage more people to share with everyone, they take the risk of exposing information that was thought to be protected.
Here's an excerpt from Zuckerberg's letter:
As Facebook has grown, some of these regional networks now have millions of members and we've concluded that this is no longer the best way for you to control your privacy. Almost 50 percent of all Facebook users are members of regional networks, so this is an important issue for us. If we can build a better system, then more than 100 million people will have even more control of their information.
The plan we've come up with is to remove regional networks completely and create a simpler model for privacy control where you can set content to be available to only your friends, friends of your friends, or everyone.
We're adding something that many of you have asked for — the ability to control who sees each individual piece of content you create or upload. In addition, we'll also be fulfilling a request made by many of you to make the privacy settings page simpler by combining some settings.
Perhaps the most useful advice from Zuckerberg comes at the end of his open letter: "the best way for you to find the right settings is to read through all your options and customize them for yourself."