Of course it isn't completely anonymous; Facebook isn't claiming that it is. Their documentation very clearly demonstrates that they are linking the "anonymous" user with a Facebook profile on the backened (evidenced by the ability to connect accounts in the future). It is anonymous to developers because they don't see the profile information until the user decides to link it. This isn't some service being offered by Facebook where we have to then worry about them "connecting the dots" in the future - they already told us what they're doing and how.

First thing I thought off. It means the application you are logging into does not know who you are but Facebook does get to know you are logging into that app.

Whichever way you look at it your privacy is better protected if one party knows rather than both parties knowing, so this is definitely a win if you already use Facebook. If you don't then the new option is irrelevant.

Facebook's pitch is to developers, not users.

Some users will think they remain anonymous when in fact they are authenticated to Facebook.

Technically speaking, of course they do. Google can also read all your emails.

But I'm not sure Facebook would bother doing so. Hopefully the privacy policy regarding anonymous login specifies what they will or won't track.

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.

Exactly. You do not know but facebook if you know who he is.