A message in this case probably doesn't mean a Tweet. A message if probably just a message on the bus/in the queue and any action on Twitter probably produces one. Like, retweet, tweet, follow, sign up, everything.

Bots, news organizations, and teenagers. ~143 messages/day per human doesn't seem unreasonable.

There are tons of bots on Twitter

A message probably counts every tweet a person views at a minimum.