There are two ways to ensure that a thread that you'd like to keep an eye on stays easily found even if it drops way down Active Conversations. One is the official route, which is to click on 'Watch', the other is to post on it, and then look for it later on Threads I'm On.
I use 'Watch' to keep threads with useful advice for the future, so currently for example there are threads about places I'm thinking of going on holiday to, about restoring decrepit old buildings, about adolescence in teenage girls, and about vegetarian food in my Watch list.
If it's a more ephemeral, 'newsy' thread I'll post on it to mark place, often with a very short, non judgy, non committed remark.
I have been guilty in the past of posting on a suspected troll thread just to see the deletion message, as sometimes they could be funny, but this isn't really a good thing to do, as it just keeps the thread higher up in Active, and also I think HQ have sussed this and don't do witty deletion messages in the way they used to.