Social proof in advertising refers to visible engagement signals on an ad: likes, comments, shares, and reactions, that signal credibility and popularity to new users encountering the ad for the first time. On Meta, ads with accumulated social proof tend to outperform fresh creatives with zero engagement because users are influenced by seeing that others have positively reacted to the content. When running the same ad across multiple ad sets, using the original Post ID preserves accumulated social proof rather than resetting to zero. Building social proof is especially important for cold traffic campaigns where the audience has no prior brand familiarity.