There's a few factors for the drop off. Some people were using automated bots to report certain things.
I was thinking that when i have seen all this numbers and for sure back in this days there was more spam as today.
A few people were probably reporting in the hope of becoming a moderator but new mods are rarely added these days so they probably gave up trying.
Reporting is also a time-consuming and thankless task with no real reward for the users so most just don't bother.
If you know where and how to report its not that much time-consuming but yeah on the the other hand a lot of time goes into research things.
I think there was also an uptake in reporting when theymos first mentioned he might add reporting badges that never materialised.
I think theymos should consider adding them them with some small perks.
Maybe theymos could ever consider some sort of financial reward or some sort of raffle that reporters get entered into.
Doesn't have to be much but maybe something would encourage an uptake.
Maybe for every x amount of reports you get one ticket into a quarterly raffle or something.
Maybe it doesn't even have to be a cash prize if you could find some people to donate some rewards, or maybe get someone else to sponsor it.
If theymos could give us the reporting stats every x amount of time maybe it could be a community driven incentive that people donate to or someone else sponsons.
For sure there was a lot of new Users when theymos have written that with the badges for the reporters , i remember.
To be honest i guess if this will be coming that this would definitely encourage some users again or even new Users to start reporting.
The other thing is as you mentioned for maybe getting an reward in any kind of form for reporting that this could be abused from others.
Creating an new Account and spam with that and with the main Account you reporting it.
But the idea itself is interesting.