Reach is the better metric for understanding audience size. A campaign with 100,000 impressions and 20,000 reach means 5 average displays per person — saturating a smaller group. The same 100,000 impressions and 90,000 reach means almost no repetition.
Platforms measure reach over a window: per post, per day, per week. Per-week reach and per-month reach are useful for content frequency planning (how often does your average follower see your content) but cannot be summed across days — the same person seen on Monday and Tuesday is one weekly reach, not two.
Organic reach has been declining for over a decade on the major social platforms as feeds prioritise paid and high-engagement content. Counting on organic reach as a primary distribution channel is risky; treating it as supplementary to paid, email, and direct traffic is more durable.