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"Thank you all for helping me through this time with your enormous love & support. Cory will forever be in my heart," read the tweet by co-star Lea Michele (@msleamichele) about Monteith's fatal drug overdose.
It was retweeted more than 408,000 times by users from 133 countries, making it the "Golden Tweet" of 2013, according to the San Francisco-based Internet company.
"Among the millions of affecting Tweets this year, there is something we call The Golden Tweet," Twitter spokesman Gabriel Stricker said in a blog post.
"It's the most-retweeted Tweet of the year -- the one that resonated most with millions of Twitter users around the world. In 2013."
A Twitter message announcing the death of "The Fast and The Furious" film celebrity Paul Walker in a car crash was second, having been retweeted 400,367 times, according to Stricker.
"It's with a heavy heart that we must confirm Paul Walker passed away today in a tragic car accident," read the tweet from @RealPaulWalker.
The third most retweeted message was fired off by One Direction band member Niall Horan rejoicing in having just turned 20 years old.
"Yesss ! I'm 20 ! Wohooo ! No more teens!" Horan chimed at the Twitter account @NiallOfficial.