For the facts and data, read Jordan McCollum's post.
I realize that a single month worth of data does not indicate a trend. For all we know the decline in Twitter volume was caused by the fear of the Twitpocalypse. Maybe everyone's away on summer vacation?
So, it's not time to active your Personal Disaster/Zombie Defense Plans. You might want to review the checklists and check your survival gear and rations.
It looks like many others are coming to the same conclusion I came to about Twitter, it's a huge waste of time. And time is the most valuable commodity I own.
Let's see how the trends play out. Time will tell.