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The words "four more years", coupled with a photo of Barack and Michelle Obama embraced in a hug, have become the most retweeted Twitter post ever.

The US president tweeted the message at 0416 GMT, and it has since been retweeted over half a million times.

It underlined the now pivotal role social media plays in informing and persuading potential voters.

As news of Mr Obama's re-election broke, there were .

The same picture of Mr and Mrs Obama was , where it also broke records by becoming the most 'liked' photo ever posted to the site.

Throughout the campaign, both candidates invested heavily in co-ordinated social networking campaigns.

This approach climaxed on results night with the Democrat using email and Twitter to announce his victory to supporters - even before he had made his speech.

Other world leaders, such as David Cameron, took to the service to offer their congratulations.

"Warm congratulations to my friend @BarackObama," the UK prime minister wrote in a message that was retweeted more than 1,500 times. "Look forward to continuing to work together."

Others, like businessman Donald Trump - who last month to release college applications - were less congratulatory.

: "We can't let this happen. We should march on Washington and stop this travesty. Our nation is totally divided!"

At the time of writing, Republican candidate Mitt Romney had not tweeted since the result. His : "With your help, we will turn our country around and get America back on the path to prosperity."

While the majority of the $2bn (£1.25bn) spent by both Mr Obama and Republican opponent Mr Romney went on television advertising and events, the internet was seen as a key battle ground for campaigning, particularly in securing the support of younger voters.

One of the stand-out moments from Mr Obama's campaign was .

The president spent around 30 minutes answering questions from the site's millions of users. Mr Obama's credibility was enhanced when he told users the Q&A sessions were "Not bad!" - an apparent reference to an .

Mr Obama later returned to Reddit ahead of polls closing to urge more people to go out and vote - as did Mr Romney on Twitter.

On polling day, social media also publicised a few problems.

One voter - also a prolific Reddit user - posted a video which he said would place a tick next to Mr Romney's name when pressing the key to vote for Mr Obama.

The machine was later taken out of service and fixed, a election official later clarified.

Well over half a million users of image-sharing social network Instagram posted images of themselves voting.

However, US law experts were quick to warn voters that in some states it is in fact illegal to take and share such images.

The legislation is designed, among other things, to prevent the possibility of vote buying.

"Display of a marked ballot to voters waiting in the queue can be used as a form of pressure to vote similarly, or to confuse voters who have difficulty reading English as to how to mark their own ballots," .

"Similarly, those who might attempt to buy votes will generally require proof that the voter has in fact been bought, possibly through display of a marked ballot."

, one voter who had used his smartphone to make notes about his chosen candidates was told he would not be allowed to take the device into the voting booth - an experienced shared by several others in the area.