Lurker
Etymology
From lurk, "to lie hidden," in continuous use on Usenet and bulletin boards since the 1980s, where newcomers were advised to "lurk before you post" — to observe a group’s norms before joining in. The noun lurker has stayed remarkably stable across every platform since.
noun · /LUR-ker/
Someone who reads or watches an online community regularly without ever posting or commenting.
What it names
It names the silent majority of any forum or chat — the many who consume without contributing. The word is useful precisely because these people are invisible in the metrics of participation yet make up most of the actual audience.
Example
I’ve been a lurker on that subreddit for years and never once left a comment.
Related idea
The inverse of a poster: same feed, opposite behavior — one absorbs in silence, the other supplies the content.