The new dashboard (which is nice!) contains a metric of user-to-user messages. These seem to include the messages that Discobot sends and that makes it fairly useless as a metric (since in small communities, new signups will dwarf PMs sent).
We launched in April, as you can see:
Feels like this metric should not include messages sent by or (solely) to users with negative ids (e.g. system and Discobot).