Just got this in from Compete.com but I had a completely different thought about the data than the author.

Where they see popularity on slowing down, I see the possibility of Twitter getting serious in their efforts to battle spam and follow bots. I also see the possibilities of lots of new and enthusiastic Tweeps finding out that automation doesn’t always work as well as advertised…

From Compete.com’s email:

Twitter Visitors Put on the Brakes

It had to happen eventually, and May was the month that traffic to Twitter.com started to show signs of fatigue (up 1% to 19.7 Million Unique Visitors). While this is not the whole story behind the emerging Social site, it is a sign that new user growth in the U.S. may be flattening. Stay tuned for more on this one going forward.