“Those do a good job of engaging audiences online, and they do a good job of letting you know how other people who have come to the webpage feel about whatever issue,” said Mollyann Brodie, the executive director for public opinion and survey research at the Kaiser Family Foundation. “But they’re not necessarily good at telling you, in general, what people think, because we don’t know who’s come to that website and who’s taken it.”

Professional pollsters must ensure that their small samples are demographically appropriate to indicate how larger groups of people think. Online polls do nothing of the sort, allowing anyone who finds the poll to vote, thus opening them to manipulation from those who would want to stuff the ballot box. Users on Reddit and 4chan directed masses of people to vote for Mr. Trump in the instant-analysis polls, according to The Daily Dot.

Even when there is no intentional manipulation, the results are largely a reflection of who is likely to come to a site and who would be motivated enough to participate in a poll. Intuitively, it’s no surprise that voters on sites like Breitbart News and the Drudge Report would see Mr. Trump as the winner, just as Mrs. Clinton would be more likely to find support on liberal sites.