If mutual friends have uploaded photos and tagged the private user, those photos will appear blurred in the thumbnail, but the tags remain. You cannot see the full photo, but you see the names of the people tagged with them. This gives you context (e.g., "Is Sarah at a party with John? You see John’s tag.").
Facebook’s privacy algorithms are updated frequently. This article is preserved for historical/educational context regarding 2021 methods. As of today, most of these "fixes" are obsolete; however, understanding them helps explain why Facebook is currently secure. view private facebook photos without being friends 2021 fix