The “microblogging” site drew 18.2 million users in May, up 6.7 percent from a month earlier, according to New York-based research firm Nielsen Online. Twitter logged 7.04 million users in February, almost doubled that to 13.9 million in March and then gained 23 percent to 17.1 million in April, when Winfrey signed up for the site on the air.

Twitter’s slowdown was expected because the pace it set in March and April was unsustainable, said Andrew Lipsman, a spokesman for Reston, Virginia-based ComScore Inc., another Web audience-ratings service. While ComScore’s final May data isn’t done yet, weekly surveys by the researcher show Twitter’s usage peaking at 6.78 million people in early May and dipping to 5.46 million by the week ending May 31.

“We saw huge growth, and a lot of that had to be trials,” Lipsman said. Some of the audience may have also shifted from visiting the site itself to using Twitter-related software tools. They let people post Twitter messages without showing up in usage statistics for the company’s home page.

In response to the report, Twitter said it’s investing in areas that will fuel growth in the long run.

“We have a lot of work to do,” said Jenna Sampson, a spokeswoman for San Francisco-based Twitter. “We are still a young company and very much focused on growth. We want to mature properly.”

It’s probably a good thing that traffic has slowed for a while, which will give the Twitter team a chance to catch their breath before the next wave of traffic hits.

Dell (Dell) says it has surpassed $3 million in sales attributed to Twitter, and the online social network, which has yet to make a profit, is watching closely.

Dell sells refurbished systems through Twitter at @DellOutlet, which the computer maker says has more than 600,000 followers. Since launching @DellOutlet in 2007, the company has taken in more than $2 million in sales directly attributed to Twitter, and another $1 million from people who start at Twitter and then move on to buy a new computer somewhere else on the Dell Web site, Stephanie Nelson, who manages the Twitter account, said in the corporate blog late Thursday night.

While not providing specifics, Nelson told USA Today that Twitter is soliciting feedback from Dell. In addition, the two companies are testing new features.

In making sales through @DellOutlet, Dell sends tweets to subscribers, telling bargain hunters the latest available refurbished systems. The company also messages couponhjs and clearance events.

The strategy has taken @DellOutlet to the top 50 most followed Twitter users, according to Dell. Such standings are tracked on a third-party Twitter monitoring service called TwitterCounter.

Other top 50 Twitter users includes brands like @Zappos, @Woot, @JetBlue and @WholeFoods. As of March, Twitter.com reached 9.3 million visitors, according to ComScore Media Metrix.