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.
I can't believe everybody's just ad-liiiiibing!