Looks like the title is incorrect, and it's actually posts per day. At least, the graph shows a few days where the number goes down, so it can't be just the total number of posts (unless there was a wave of deletions outpacing the posts).
Edit: according to other people's comments, it is actually total posts, not posts per day. So my above comment seems to be incorrect.
Or botted contents going, or instances going offline. I'm pretty sure it's total. I think world and ml have around 500k posts. There is no way it can be a daily contents when those 2 are the most active.