President Obama answering questions on Reddit in August. Image via Reddit.

Barack Obama certainly knows his audience.

"Think of it as upvoting," Barak Obama said in a Reddit post at 5:30 p.m. ET tonight as he returned to the social media site to urge users to get out and vote for him.

"I want to thank you all again for the reception you gave me in August for my AMA. Good questions. Definitely not bad," Obama, whose username is PresidentObama on the site, said. "I'm checking in because polls will start closing in this election in just a few hours, and I need you to vote."

In late August the president hosted a Reddit AMA (Ask Me Anything), taking questions from users on the site. In that session he fielded questions on internet freedom, the space program, the most difficult decision of his first term (the surge in Afghanistan) and the White House beer ("it's tasty"), among other things. Reddit is built like a forum or commenting system; a user can start a thread and then others can comment and talk about it below.

Reddit has surged in popularity over the last couple of months. It has also been at the center of political discussions as Reddit's co-founder has tried hard to start conversations about the role of the Internet in American life on the site and with an "Internet 2012? bus. The bus went from Denver to Danville, Ky., during the debates. Reddit co-founder Alexis Ohanian told ABC News all about the bus tour in an interview last month.

"If you've voted already, don't stop there - spread the word to your friends, roommates, and neighbors," he concluded in his post tonight. "Think of it as upvoting." (Upvoting, for non-Reddit users, is a way of liking or promoting a post.)