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7 year oldChelsea Manning, whose sentence was commuted by the outgoing president, thanked him "for giving me a chance".
The 29-year-old transgender soldier, born Bradley Manning, gave embarrassing documents to Wikileaks in 2010.
At President Obama's final press conference on Wednesday, he declared that "justice has been served".
She will be freed from the Fort Leavenworth military prison in May - 29 years head of her scheduled release in 2045.
Mr Obama's last minute commutation quickly drew criticism from Republican circles.
A spokesman for president-elect Donald Trump said he "is troubled by this action. It's disappointing and it sends a very troubling message when it comes to the handling of classified information".
Republican Speaker of the House Paul Ryan "called the decision "outrageous".
"Chelsea Manning's treachery put American lives at risk and exposed some of our nation's most sensitive secrets," Mr Ryan said.
Manning does not have access to the internet, as part of the condition of her imprisonment.
However despite that, she has been tweeting from @xychelsea since April 2015.
In a handwritten letter she released to explain her social media use, she told how she calls a friend on the phone who transcribes what she says "verbatim".
Her thank you was addressed to @BarackObama, rather than the President's official @POTUS Twitter account, which Mr Obama will hand over to Mr Trump on Friday when he officially becomes the nation's 45th president.
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