Facebook users can now officially endorse a candidate in the United States presidential election.

But you better be ready for an argument if you do.

The social media giant added the feature with less than a month to go before America heads to the polls on November 8.

Users can write a message to go along with their endorsement, and control which of their friends can see it.

By visiting the page of a candidate, you can also see which of your friends has endorsed them.

So far Facebook has made the option available on the official pages of Donald Trump, Hillary Clinton, Jill Stein and Gary Johnson.

You're probably going to end up in an argument. That's not a bad thing

Curtin University senior lecturer in internet studies Tamar Leaver said the function was typical of the way Facebook had acted around the election.

"I think they have been a central place for discussion about politics. We can undeniably say that Trump is the social media candidate," he said.

Dr Leaver said by choosing to share the endorsement, users would need to be aware of the consequences.

"Anyone that is going to endorse knows that they're probably going to end up with an argument with someone on the other side of the political spectrum," he said.

"But I think that's not necessarily a bad thing.

"It does ask people to say why they believe this candidate is the best one for them.

"That might actually start some useful conversation. I do think its better than just those graphical overlays that we saw on avatars."

Dr Leaver said the people most likely to use the function are already politically active on Facebook.

Basically, you aren't likely to find out your high school friend is a Donald Trump supporter from just a Facebook endorsement.

They've probably been posting about him for months.

A recent Monmouth University poll found that 7 per cent of voters had lost or ended a friendship because of the presidential race.

Broken down by candidate, 9 per cent of Clinton voters and 6 per cent of Trump voters had lost a friendship over the campaign.

Facebook can have a 'tremendous impact' on a campaign

It's not the first time Facebook has injected itself into the US election campaign.

It's fair to say Trump and Clinton won't be Facebook friends after the election. ( Reuters )

From September 23-26, American Facebook users had a banner at the top of their feeds encouraging them to register to vote.

The Centre for Election Innovation and Research found the push had a "tremendous impact" on the number of people registering to vote.

Election officials in several states also credited the surge in voter registrations to the Facebook push, according to The New York Times.

Dr Leaver said the endorsements function is not likely to have the same impact.

"It's pretty clear from most people's Facebook feeds, if they are going to be involved in political discussion, they are already there," he said.

"This is more giving people one extra way of expressing very clearly their political views."

Endorsing a PM on Facebook wouldn't be quite the same

Dr Leaver said most Australians don't engage in politics the way everyday Americans do.

"Talking politics here is still seen as a dirty thing to do," he said.

"People that are actively talking about politics in Australia often are doing it somewhere other than Facebook. Twitter and the #auspol hashtag is much more active."