Compete.com just released data for the month of July and it appears that Twitter’s growth rate—measured as unique visitors—has taken a big hit.

While Twitter grew 16.6% in June, the site slowed to a snail’s pace of just 1.25% in July.

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What does this mean for the previously declared fastest growing social media site on the internet by Nielsen just a few weeks ago?

But—to put things in perspective—this is the second time Twitter has slowed since January. In May of this year, its growth went from 38.56% in April to just 1.47%.

One thing we can be sure of is Twitter’s 949.6% growth from July 2008 to July 2009. Impressive—but then again—even this is down from a previous 1,164% growth from June 2008 to June 2009.

What’s going on here? Is Twitter beginning to level out?

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The graph below speaks for itself, but I’ll tell you anyway: Twitter is at the 6 million unique visitors mark for January. Good news for Twitter researchers like myself. A recent study by Pew Internet also notes Twitter’s surging growth:

As of December 2008, 11% of online American adults said they have used a service like Twitter or another service that allowed them to share updates about themselves or to see the updates of others.

Twitter, rock on.