Of course most people won't leave Facebook, and if anything they will add more users than those that leave.
As I look through the changes (see Facebook's notice here with links to the details), I think there are three things to know...
1. Facial recognition and tagging suggestions. There has been buzz for years about the use of facial recognition - scanning profile and other uploaded pictures to try to identify individual facial characteristics, then using that information to find the same face in other photos. Now Facebook says they "may" start doing this.
2. Teen users implicitly give their parents consent. Facebook users are supposed to be age 13 or older. The new policy states that when a teen signs up they give both their own, and their parents', consent for the use of their information. For more info, see NPR's coverage here.
3. Your info can be used in ads. This has also been an issue in the past, but it is now clarified. I think TechCrunch puts it best in their coverage:
The rewritten section about ads is clear: “You give us permission to use your name, profile picture, content, and information in connection with commercial, sponsored, or related content.” In other words, Facebook can use anything you have uploaded in its advertising algorithms and systems.These changes, like all Facebook's changes, are not really surprising. They can do what they want. As I've discussed here and here, you are not a Facebook customer, you are the product. You can roll with the changes, or leave.
For example, it could be very handy to have Facebook recognize, and notify you, whenever a picture of you is uploaded.
The key is, as always, that you must regularly review and update your privacy settings. Here are some great pages with advice on settings. Be sure to check your settings today!