Growth for Twitter, the superhot San Francisco microblogging startup, slowed significantly in May, according to the digital traffic measurement firm Compete.

Monthly unique visitors to Twitter rose a mere 1.47 percent, or 285,333, to 19.7 million between April and May, according to Compete. The number of visits increased just shy of 7 percent.

That compares with an increase of monthly visitors of 5.4 million during March and 6.1 million in February.

Compete’s figures follow closely on the heels of a controversial Harvard Business Review study that looked at 300,000 Twitter accounts and found the top 10 percent of Twitter users accounted for over 90 percent of tweets, as postings on the service are called, while over half of the account holders tweeted less than once every 74 days.

On a typical online social network, the top 10 percent of users account for 30 percent of all production, the study asserted.

At the same time, however, Twitter is proving its worth to some corporate users, even though Twitter itself has not announced how it is going to try to make money off its free service.

Dell Computer is reporting that since 2007 more than $3 million in sales have come through its @DellOutlet account, either through direct purchases or by channeling traffic to other Dell sites. @DellOutlet has more than 600,000 followers.