Four times the number of votes
In a Facebook experiment published in Nature that was conducted on a whopping 61 million people, some randomly selected portion of this group received a neutral message to “go vote,” while others, also randomly selected, saw slightly more social version of the encouragement: small thumbnail pictures of a few of their friends who reported having voted were shown within the “go vote” pop-up.
The researchers measured that this slight tweak — completely within Facebook's control and conducted without the consent or notification of any of the millions of Facebook users — caused about 340,000 additional people to turn out to vote in the 2010 U.S. congressional elections.
(The true number may even be higher since the method of matching voter files to Facebook names only works for exact matches.)
That significant effect—from a one-time, single tweak—is more than four times the number of votes that determined that Donald Trump would be the winner of the 2016 election for presidency in the United States.