Repeating an effort that in past elections has boosted real-world voter turnout, Facebook is expected to announce Monday that it plans to post get-out-the-vote messages to the tens of millions of voting-age Americans likely to log in to the site this coming Election Day.

This follows the groundbreaking finding, published in the journal Nature two months ago, that a particular type of message posted on Election Day in 2010 boosted actual turnout by at least 340,000 votes (see “How Facebook Drove Voters to the Polls”).

James Fowler, a political scientist at the University of California, San Diego, who participated in the 2010 study, says that this coming Election Day, he expects more people to log in to Facebook—and thus see any messages that encourage voting—than did two years ago. “Last time, 61 million people logged in,” he says, referring to U.S. Facebook account holders of voting age. He says his high-end estimate for Tuesday would be about 100 million users.   

However, any ensuing boost to real-world voting might actually be smaller than it was in 2010, because this is a presidential election year, when larger fractions of voters are already committed to going to the polls anyway. In addition, this year more voters are voting early than in previous elections.

Facebook has experienced rapid worldwide growth in recent years, from 100 million monthly active users in August 2008 to a billion in October of this year. However, the growth of the U.S. component has slowed sharply. Current U.S. membership is about 160 million, up slightly from last year.

Increased voter turnout tends to favor Democratic candidates. Nationally there are far more registered Democrats than Republicans—72 million to 55 million. About 42 million are registered as Independent. However, those involved with the Facebook push say there’s no evidence that their effort would favor one presidential candidate over the other.

Facebook’s own get-out-the-vote drive, of course, will be just one of the myriad appeals expected to appear on the site on Election Day. Both President Obama and Mitt Romney are expected to furiously campaign to get their known supporters to vote—and to encourage voting.

The Obama campaign, for example, has put out messages to people who have downloaded the president’s Facebook campaign app, saying “Early voting is here. Tell your friends!” Anyone who did download Obama’s app also gave permission for the app to see his or her friends list—as well as the locations and ages of those friends—so the list likely consists of possible supporters in swing states (see “Facebook: The Real Presidential Swing State”).

Clicking the message brings up a list of friends who might be interested in knowing this. Fowler, who received such a message, described it this way: “It looked like they were weighting friends in competitive states and by how close they were to you.”

In Facebook’s 2010 get-out-the-vote effort, the company posted special reminders that said: “Today is Election Day.” These messages included a button to click to indicate if you’ve voted. The paper in Nature reported that when these reminders included face pictures of friends who had voted, it drove an additional 340,000 voters to polls, based on voting records that the researchers had access to.

Beyond seeing posts that encourage voting, Facebook account holders (and Internet users generally) will see a barrage of last-minute online ads, as campaigns continue until the last penny is spent or the polls close, says J.D. Schlough, a political strategist. “The worst thing that can happen for a campaign manager is to end a campaign with money in the bank, and lose,” he says.

Obligingly, Google and Facebook and other ad networks are providing a dizzying array of advertising tools and strategies, including ones that track people by their behaviors and interests. Another way the campaigns target voters online is by hiring companies like Audience Partners, which matches real-world voter records with their computer addresses, allowing targeted ads (see “Campaigns to Track Voters With Political Cookies”).