For all that G+ is a horrible service, the fact that commenting on a post is effectively a subscription to that post has been among its more attractive elements. If the moderator of that post kicks crap (spam, shitposts) to the curb.

It's lead to the convention of "/sub" (or as I prefer, "/surveil") as an indicator of interest.

But a good conversation can remain alive for days, weeks, months, or even years.