Memes have velocity, amplitude and duration.  Natural memes are an S-curve, like this graph of the "Supernatural" television series:

Look at this Greta Thunberg meme.  There's no growth phase, it's just instantly everywhere at a huge amplitude.  

Now look at the past 24 hours.  The growth phase is 14 hours and by 7:00 am this morning there's pretty much zero public interest.

Compare that to the Pope's death in 2005 which didn't reach maximum amplitude for five days.   People are busy with other things in their life, so it takes time for any information to sink in.  Generally, the most drastic and personally relevant memes travel faster.

The Thunberg meme is probably an agenda driven by the media oligopoly, deliberate propaganda to trick you into some behavior that you need to be tricked into.   A manipulated meme can't gain traction because nobody is interested, so regardless of how it is pumped, it just collapses again.  I'd bet money that the "Thunberg"  meme will resolve into this pattern.