It's the same problem as Facebook and other social media: people present a curated persona, and ignore that they're only seeing everyone else's curated persona as well.
Ad marketing is already well into figuring out how to manufacture "authenticity".
You can have bits of social satisfaction from hearing 'hello, how are you' from a store clerk or from watching people interact on TV. Definitely you can get it from getting responses to you anonymous or curated speach on the internet. Of course its not the same thing as honest interaction with your friend or loved one but it doesn't mean it does nothing for you.
Please notice they made people feel isolated by not passing a ball to them in a computer game.
Social circuits in the brain are so low level they can be activated by most superficial stimuli.
Social isolation does not make you extremist on itself - contact with extremists that recruit you make you extremist. Social isolation make you more vulnerable to extremists.
Three is just a number if you don't know the sample size. If it's 3 out of 3 million and the US had 600 mass murderers out per 300 million population then 8channer is twice as safe as average Joe. (Made up numbers to illustrate the math).