Selection bias is a drift in statistical results as a result of sampling mechanisms, to the point that the selected population no longer effectively correlates with the larger population against which an inference may be erroneously made.
As pointed above, it is erroneous to assume opinions visible on a website such of this are representative of how humans think.
There are a large number of selection steps which bias the results.
Opinions are only from people who are currently alive. Those of the past and future are unable to have their opinions heard directly.
All opinions visible on Tumblr are from people who are aware of Tumblr and chose to use it in a language that is mutual with the viewer. Vast swaths of humanity are unaware or uncaring of the website, or are communicating in a different language.
All opinions visible on Tumblr come from the subset of users who post them. As with many websites, a large portion of users exclusively lurk or reblog, rather than share their own opinions. Only a certain subset seeks to have their thoughts known.
All opinions visible are those that have been judged worth sharing. Individuals may self-censor or find their own opinions too boring to be worth conveying.
For an opinion to be visible on Tumblr, it must either come from the subset of individuals that a user has chosen to follow (a curated group), be reposted by said curated group, be tagged with a tag a user follows, or be amplified by enough people to attract general attention by a given individual. This introduces a bias towards opinions that one's curated groups believe or choose to amplify, opinions that match a tag a user curates for themselves, or opinions that are amplified.
It would be a fallacy to believe that after all these non-random selection phases, any remaining opinion one sees is representative of humanity at large.
But one can make non-erroneous judgements such as "this is an opinion that exists in humanity" or "this is an opinion shared by this particular subculture on this website."
In the early twenty-first century, even when aware of Selection Bias, many people do not always remember to apply it in their assessments. It is a common problem for people to forget that the information available to them is curated, either by their own efforts of via the effort of other parties/algorithms.
