The number of posts WordPress.com users are publishing.
In addition to blog posts, WordPress.com users can publish web pages as well. Pages are separate from a normal blog chronology but can still be easily managed in the interface (you are reading such a page right now). This allows people to easily create an entire site full of hierarchical content about whatever they like.
These are the legit reader comments that make it past Akismet every month.
People love uploading files.
We support standard blog posting APIs like XML-RPC, and this is the number of posts per month that use those APIs.
Think we should be tracking something else, or curious about more stats? Let us know.