Users who register/login via http://facebook.stackoverflow.com get their facebook profile photo as their default avatar. However, the avatar is not copied to SO servers or imgur and thus the original URL is kept. This URL compromises the privacy of the user as it contains their facebook user ID which everyone can easily extract and then view his profile.
The image is also not visible at all to those who have Facebook blocked for their own privacy (by extensions such as Ghostery), or due to network policy (e.g. those accessing Stack Exchange from work).
With the new avatar system it would be much better if the facebook photo was stored like a manually uploaded avatar to avoid this problem.