Users who register for their account via Facebook get their Facebook profile photo as their default avatar. However, the avatar is not copied to SE 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 the 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 browser features like Firefox's Tracking Protection), or due to network policy (e.g. some work and school networks block Facebook).
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.