I'm hypothesising that this is a bias people have when reading posts in the internet. People are susceptible to it to different degrees. And your statement doe

I'm hypothesising that this is a bias people have when reading posts in the internet. People are susceptible to it to different degrees. And your statement does show this bias in a different way. You comment on the 10 being too small a sample size. That implies that if there were more posts, say 90 vs 10, you would draw some conclusions about the question "Do people like the new UI?". In the same way you're saying that the sample would be biased because someone posted on YouTube, I'm saying it will _always_ be biased, regardless of size, because the post itself (and all posts) attracts a biased sample. A larger sample is good for a random sample, but I think posts attract a sufficiently biased sample by their very nature, so increasing the sample size won't help.