It is in their direct interest to do so. They could use that information to better target ads, and to keep an eye on the popularity of potential acquisitions.

It's not in their interest to get sued for violating their privacy policy.

In some contexts, Facebook is "reading" the offline behavior of users. Why would they not take advantage of online data, when it could generate revenue in some contexts?

That's true, but there is a difference between offline/not-logged-in and actually "anonymous". If they're offering something which they advertise as being anonymous, there is an explicit understanding that things you do won't be reflected when you login to your primary accounts.

They may very well take advantage of it though; they certainly are able to do so legally. We'll see what the policy looks like.