Delete unused Facebook user avatars

What extension are you integrating with? The images get stored in the temp folder for your Joomla installation. You're free to delete any of the images in that directory without any problems. If you're talking about the images in one of your extension's (Community Builder, JomSocial, etc) avatar folder, that would be different and harder to do as that extension should be managing the avatars on their own.

Thanks,
Alex