If traffic jumps but users don't return, that's usually surface-level attention, not true spread. Strong signals include rising returning-user cohorts, growing direct searches for the product or topic, and steady engagement like saves, shares, and comments that continue after the peak. You also see referral loops where new users bring others without paid pushes, plus increasing session frequency over time. If DAU/MAU stabilizes instead of collapsing, that's another clue. Cohort retention curves flatten instead of dropping sharply. Together these patterns show sustained interest rather than one-off curiosity over time as well.