Although some of it is due to multiaccounting, I'm guessing that a lot of it is:

Unable to gain traction with their early posts so they give up.

Combine this with, earlier users also getting frustrated at lack of progress, and it could explain the falls in participation.

Why? Because that recent burst in sign-ups coincided with articles purporting to show how to game the system to get more rewards. Unsurprisingly, a lot of the whales didn't co-operate because no-one likes being played. So, (the bulk of) the folks who tried it out didn't succeed, got frustrated and gave up.

(I could be wrong: that hope-to-frustration fall-off could have been experienced by multi-accounters trying to game the system.)

Whatever the reason, it's just a bump in the road.