According to research by the firm, which develops online metrics software, while the rate of new user growth peaked last July at about 7.8 million a month, that number has dropped to about 6.2 million new users a month now.
The past six months, the research also noted, have shown a steady falloff in the number of new accounts.
"When you look at new account registrations, no one can deny that Twitter is still growing like a rocket ship. That's good," said Robert J Moore, CEO and founder of RJMetrics.
"However, upon closer inspection, the rate of new user signups has dropped meaningfully from its peak and many new users never do anything with their accounts. That's bad. Furthermore, the percentage of accounts sending out tweets has steadily declined over the past six months. That's worse."
Actually, the study shows that a lot of Twitter accounts aren't active, and the number of accounts that sent even one tweet in all of last December hit an all-time low.
According to the findings, only 17 percent of all Twitter accounts Twittered last month. That's down from more than 70 percent in early 2007 when Twitter was a fledgling company with far, far fewer users.
However, Moore pointed out that because of Twitter's "rapid user growth", even with only 17 percent tweeting last month, that still adds up to more Twitterers than ever before.
Twitter, according to the report, has between 10 million and 15 million active Twitterers.
Today's study echoes a report released last week. HubSpot, a web analytics company, noted in its own report that the number of users joining Twitter started to drop off dramatically last fall.
While Twitter grew rapidly over the past couple of years, HubSpot said in its latest 'State of the Twittersphere' report that the company's growth rate dropped to 3.5 percent in October, compared to 13 percent just seven months earlier.
This slowing growth rate stands in stark contrast to the micro-blogging site's staggering growth numbers just a year ago.
ComScore, a web analytics company, reported that the number of people using Twitter in February 2009 had jumped a dramatic 700 percent compared to the same month in 2008.
Twitter also saw a 131 percent increase in US visitors between February and March of last year, according to another report from ComScore.
Twitter had 9.3 million visitors in March of 2009 - a whopping five million more than it had the month before.
According to today's RJMetrics report, people who have joined Twitter aren't creating much of a presence there.
The average Twitter user has 27 followers, which is down from 42 followers in August, according to the new study.
About 25 percent of users - up from 20 percent last August - have no followers at all. Upwards of 40 percent of users only have between one and five followers.
"A third of Twitter's user base has joined up in the past four months, and we know that users acquire more followers the longer they are on the system," said Moore.
However, he added that it's "impossible" to tell at this point if so many users have few followers because they're new to the site or if they're simply not engaged.
And a lack of engagement is showing up.
The study noted that about 80 percent of all Twitter users have tweeted fewer than 10 times, up five percentage points from just five months ago.
Moore pointed out in the report that if new Twitter users stick with the micro-blogging service through just the first week they have a much higher rate of engagement with the site over time.