"I'm a Voter." The declaration is popping up all over Facebook on this Election Day, as users let their world of acquaintances know they performed one of their basic American civic duties, just by clicking the "I'm a Voter" button. In doing so, users of the social media site enter into a grand experiment they may not even be aware of.

The "Voting Across the Nation" feature is a Facebook experience unique to the election that allows users to see how many fellow Facebookers have voted via the use of a map updated in real time as turnout increases and the 2014 midterm votes are cast. One accompanying graph shows the number of ballots cast per hour -- more than 400,000 per hour during an early peak shortly after polls opened on the East Coast -- while another compiles data on who the voters are, plotting the data by gender and age. The data behind the experiment is derived from a non-scientific self-reporting process that essentially scans the demographics of each Facebook user who clicks the "I'm a Voter" button.

"This map represents people who shared with their friends on Facebook that they're voting in the 2014 U.S. election," Facebook explains on the "Voting Across the Nation" site. "The information displayed here has been aggregated, and personal information like names have been removed. The shade of the map corresponds to the total amount of people who have shared that they're voting, while the size of each burst represents sharing activity right now."

There is some controversy surrounding the project, which prompts Facebook users at log-in as follows: “It’s Election Day. Share that you’re voting in the U.S. election and find out where to vote.” It goes further by telling users how many people have pressed the button so far (nearly 4.5 million as of 3 p.m. Eastern time Tuesday) and the names of users' friends who have already reported that they cast ballots.

The results of the 2010 version of the Facebook "I'm a Voter" button were used as the basis for a study that "found that the voting button could shape who actually voted to a significant degree: If you’re told your friends have voted, you’re 0.39 percent more likely to vote than someone who hasn’t. Facebook believes that in 2010, its election-day module was responsible for more than 600,000 additional votes," according to the Atlantic. The issue some see is that the type of people Facebook may draw to polls likely skews young, female, urban and tech-savvy, a group that tends to lean left on policy issues.

"Social networks skew young and female, two reliably progressive-leaning demographics. Even if Facebook distributed the button equally to its users, it might still bring more liberal users to the polls than conservative ones," the Atlantic points out.

Beyond that, Mark Zuckerberg's ubiquitous social network has varied the look and placement of its voting button in ways that could further skew turnout, and thereby have further impact on electoral results. In 2012, Facebook did not show all users the same button in the same way. Instead, it showed different wordings for different people, put it in different places on different users' pages, and more, according to Mother Jones.

"Most, but not all, adult Facebook users in the United States had some version of the voter megaphone placed on their pages," Mother Jones reported about the 2012 rollout of the button. "But for some, this button appeared only late in the afternoon. Some users reported clicking on the button but never saw anything about their friends voting in their own feed."

The company said that the different ways the button was displayed to various people had no impact on the election. "Some of [the different versions] were more likely to result in clicks," Michael Buckley, Facebook's vice president for global business communications, told Mother Jones. "But there was no difference…in terms of the potential ability to get people to the polls. Bottom line: It didn't matter what the button said."