Surely all this would give us is the opinions of people who like blockchain, and I doubt that such people are representative of the wider population.

As far as I'm aware Mori and PEW polls on public opinion are independent and while they may ask a very limited range of questions, at least they try and access a broad range of people.

Yes they have been terrible at predicting election results recently, but I think that's just because of the nature of that particular topic - it's got harder to predict results because people are less certain about who they are going to vote for! Where their polls on other topics are concerned I think they're pretty valid...

I think fake news is either a matter of private companies or interest groups deliberately designing research polls to get the results they want and/ or publishing that and other data selectively and/or out of context.

Source checking and refuting is how you combat fake public opinion!