Learn English – Antonym for Trending (in the Twitter sense)

When a wildly trending topic (or "trend" as it is called) has quickly lost popularity (popularity in terms of how "in the moment" something is) it is retroactively labelled a fad, and is considered "dead". Some trends become what are technically called memes, which are defined as:

1. An idea, belief or belief system, or pattern of behavior that spreads throughout a culture either vertically by cultural inheritance (as by parents to children) or horizontally by cultural acquisition (as by peers, information media, and entertainment media).

In this case when I say "become what are technically called memes" I mean ideas that are culturally widespread due to cultural inheritance (like the idea of the American Past-time being Baseball, which is a very old idea) vs whatever the latest YouTube video that is at present popular and then in a week fades to obscurity. Such "vertically spread ideas" don't seem to have a name, though you could possibly describe the word stereotype. Even then though, stereotype doesn't exactly work either.

Back to your question, as far as I could find, there is no accepted term for a topic (or "trend") that is losing popularity. You could say "the topic is trending downwards, trending towards obscurity, fading (I'd guess this simply becuase it's close to fad), is becoming unpopular, or some other related 'nobody cares' term.", but nobody really talks about trends in that way.

This lack of wording is possibly due to the fact that the very nature of modern pop culture with the internet is very present tense oriented and people simply just forget about things and move on to the next so quickly people don't realize something is maybe becoming unpopular, it just suddenly is. Sometimes trends can be "revived" but such happenings are rare and usually short lived.