“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 use scientific statistical methods to make sure that their small random samples are demographically appropriate to indicate how larger groups of people think. Online polls do nothing of the sort, and are not random, allowing anyone who finds the poll to vote. They are thus open 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 surveys, according to The Daily Dot. Similar efforts were observed on Twitter and other sites.

Even when there is no intentional manipulation, the results are largely a reflection of who is likely to come to a particular site and who would be motivated enough to participate. Intuitively, it’s no surprise that readers of 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.

It’s not just in politics; take the question of identifying the best undefeated team in the N.F.L. after three weeks. The New England Patriots and the Denver Broncos are likely the most obvious candidates for football fans, but when a news site in Philadelphia asked the question on Sunday after a big win by the Eagles? You can guess what happened.