<b>Every join cohort has a half-life</b>

<b>Every join cohort has a half-life</b>

Servers report headline member counts. The cohort lens tells a more honest, more useful story.

<b>What the data shows</b>
When joins are bucketed by month and tracked, most communities show a brutal early cliff: a large share of any cohort goes inactive within the first 7 days, and the curve then flattens into a slow-decaying tail. A useful summary statistic is the cohort half-life — the time for a join-month group to lose half its actives. Healthy topic servers often land somewhere in the multi-month range; servers riding a viral spike can have half-lives measured in <i>days</i>.

<b>Why it happens</b>
The early cliff is filtration — wrong-fit joiners leave fast. The tail is genuine attrition of fit members and is the number worth defending. Spike-driven growth inflates the top of the funnel with low-intent joiners, which is why raw member count and viral moments flatter a server that's actually bleeding.

<b>The caveat</b>
"Inactive" is a definitional choice (no message? no visit? no reaction?), and different thresholds move the curve a lot. Without server-side analytics, most owners can only approximate this.

Open question: is the early cliff worth fighting at all — or is it healthy filtration that you should optimize the <i>tail</i> around instead?