It’s been an explosive year of growth for Twitter, with celebrities and the media embracing the microblogging social network. Oprah featured Twitter on her show and crowned Ashton Kutcher the King of Twitter after he became the first person to gain 1 million followers.

But new Nielsen data shows that growth tapered off in the month of May, when traffic “only” grew by 7% over April.

Don’t get out your Kleenex, though. Twitter’s year-over-year growth was a whopping 1,448% year-over-year, growing from 1.2 million uniques in May 2008 to 18.2 million in May 2009.

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