You describe the usage of the term in your language:
Because people always go to the same forum, they thought everyone thinks everything the same way as they think.
The term echo chamber describes what you are talking about exactly. Wikipedia says:
In discussions of news media, an echo chamber refers to situations in which beliefs are amplified or reinforced by communication and repetition inside a closed system and insulated from rebuttal. By participating in an echo chamber, people are able to seek out information that reinforces their existing views without encountering opposing views, potentially resulting in an unintended exercise in confirmation bias. Echo chambers may increase social and political polarization and extremism.
They begin the article by talking about news media, but the phenomenon is also very prevalent in social media where users have "feeds" that are curated by themselves or by the system, e.g. Twitter, Reddit, Facebook, etc. People may also choose to visit specific websites and forums that reflect their existing views.
This webpage from the Digital Media Literacy project discusses echo chambers.