This question is an exact duplicate of:
So I had a user today whose profile image wasn't loading. It turns the user's profile image was linked to Facebook since they'd signed up using Facebook auth, and the profile image had been pulled over as a link back to Facebook. Ghostery was then blocking the image request since it uses the graph api.
Privacy issues aside, a lot of people browse StackExchange from/for work, and Facebook et al are blocked in a great many workplaces - other people in this situation also saw no avatar at all for this user.
I would suggest that when a user is created via Facebook, the profile image is copied over to stack.imgur and then served from there, or the profile image is not taken at all, or even having a gravatar as a backup for those for whom the off-site image is not available.