Today (March 19, 2020) first time count of registrations per round became greater than fantastic 18000. To be presice it was 18180 (just one more and it would be palindrome). But only one (!) year ago it became more than 10000, so, it increased almost twice. Yes, we are sick of sports programming and it's contagious.
Less words, more pictures:

Three notable things:

• March, 2014: Black day. I've already forgot about it. Now with even with more than 10k participants codeforces has almost no queue in the beginning of contests;
• September, 2015: The Second Revolution of Colors and Titles;
• Upper envelope of this graph is combined contest, such as Hello/Goodbye/Global rounds. Some detailed graphs (count of rounds in brackets):

There were 217 rounds more, for which it's difficult to determine type in code by its title, e.g. sponsored contests.
I'm glad to see that codeforces audience became so big!
If you have any ideas to share, you're welcome!

UPD: Added graphs with participants count (who submitted at least once during contest). Also added graphs with percent of participants to registrants). On Div1 graph two lowest points are mirrow of team contest, which have small percent of participants by definition and Technocup Final 2019.