That’s only partially true.
Online polls can be highly biased. Online polls can be highly accurate. Here’s insight from Pew Research in 2010:
“By contrast, most online polls that use participants who volunteer to take part do not have a proven record of accuracy. There are at least two reasons for this. One is that not everyone in the U.S. uses the internet, and those who do not are demographically different from the rest of the public…Keep in mind that there is nothing inherently wrong with conducting surveys online...And some online surveys are done with special panels of respondents who have been recruited randomly and then provided with internet access if they do not have it.”
In 2010, “not everyone is on the Internet” carried more weight than in 2017 when almost 90 percent of the country has Internet access. Additionally, there is selection bias in telephonic surveys as well e.g. what demographic in the US still has landlines, what demographic in the US will pick up a random number on their cell, what demographic will actually participate in a telephonic survey if they do pick up the call, etc.
No poll is perfect and I wouldn’t stake the farm on a single poll of any kind, but pretending this poll is meaningless when no experts have attacked it seems suspect.
