It was only two months ago that we noted that in just one month Twitter had added 5 million visitors to its site, doubling traffic. Now, data from Compete and Quantcast shows that Twitter traffic flattened last month—and may have even fallen.

Granted Twitter still has traffic that any big media site would kill for. (To make just one comparison: It’s still greater than traffic to both the WSJ and NYT sites). But it begs the question of when Twitter is going to roll out some new features that would improve the site’s user experience (and thus it’s well-documented user retention problem). Executives have said they are looking into it. John Battelle, who pulled the Quantcast data, says not to worry, predicting that “Twitter will address this issue, and growth will resume, but at a more moderate and sustainable pace.”

The bearish charts, after the jump.

Quantcast, followed by Compete:

• Twitter Added 5 Million Users In March
• Twitter Executive Talks About Keeping Tweeters From Leaving
• Another Cause For Concern At Twitter Headquarters