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

Check out that Greta Thunberg meme, long-term.  There's no growth phase, it's just instantly everywhere at a huge amplitude.  

Now look at the short-term graph.  The entire meme growth phase is 14 hours.  There was pretty much ZERO interest at 6:59 am this morning.

Compare that to this meme of the Pope's death in 2005.   That meme didn't reach maximum amplitude for FIVE days.   People are always busy with other things in their life, so it takes time for ANY information to sink into the general public.  Generally, the most drastic and personally relevant memes travel faster.

The Thunberg meme is probably a coordinated effort by the media oligopoly to push an agenda.   It's deliberate propaganda to trick YOU into some behavior that they believe you need to be tricked into.   A manipulated meme can't gain traction, regardless of how it is pumped, so it rises and then drops again.  I would bet money that the "Thunberg"  meme will resolve into this pattern over the next one to two years.