already announced who they cast their ballot for -- or planned to -- on social networks flooded with "I voted" posts, clicks and photos.
"Just cast my vote & my grandpa would be so proud!", @She_Weezy2012 tweeted, posting a picture of an "I Voted" sticker on her grey jumper, complete with the #ivoted hashtag, which was being used by hundreds of Twitter users.
Wearing aviator sunglasses and a black cap, singer Lenny Kravitz also waded into the fray, sticking his "I voted today!" badge on his finger in a photo on his Twitter account.
In the run-up to Tuesday's election, campaigns, celebrities and others have used social networks extensively to try to persuade people to vote and even beat the record 2008 turnout, when two-thirds of US voters cast a ballot.
Facebook itself posted messages at the top of people's news feeds on Tuesday showing users which friends were voting in the election and urging them to do the same by clicking an "I'm a voter" button.
On a separate page, a real-time map of the United States lit up in various areas as soon as a person clicked the button, with a counter clocking-up the hundreds of thousands of Facebook voters (http://www.facebookstories.com/vote).
In its study, the Pew Research Center found that 22 percent of respondents in a representative sample of 1,011 adults said they had let people know who they voted or planned to vote for on social networking sites.
The survey also found some 25 percent of Barack Obama supporters had publicly acknowledged their choice, while 20 percent of Mitt Romney backers had done so.
Nearly one-third of voters, meanwhile, had been encouraged to vote for Obama or his Republican challenger Romney via posts on social media, while one-fifth had tried to convince others to cast their ballot on social networks.
On Twitter, photos abounded of people proudly sporting their "I voted" badges and some also posted shots of their ballot papers -- which is illegal in some states.
Others put up photos of them voting with their children -- an initiative backed by First Lady Michelle Obama, who has encouraged Americans to take their kids to polling stations so they get an idea of the workings of democracy.
"Happy Election Day" was trending nationwide on Twitter, as was the hashtag #ivoted. The social network also set up a dedicated page for the US vote.
Picswitch, a website that allows users to customize their Twitter profile pictures, had a large selection of logos provided by Obama's campaign team that people were adding to their photos.
The most popular was the generic "I voted" banner with Obama's campaign logo. But dozens of people had also chosen a shot of the US president taken from behind, with the caption "I've got his back."
Many also checked in on the location-based social network Foursquare to pinpoint exactly where they cast their ballot, and 34,700 photos had already been posted on photo-sharing platform Instagram with the hashtag #ivoted.
Google also got in on the act, enabling users to click on the search engine's Doodle -- made out of ballot papers with its second 'g' falling into a voting box sporting the US flag -- to find their nearest polling station.