It is more true online, and for a lot of reasons.
I meet someone at a party and we become Facebook friends. I then post some sort of political article or comment to my feed. They see it, disagree with it, and (likely) either comment on it in a non-polite manner, or stay silent, but either way categorize me in their head as a crazy right/left winger. Why? Because these are what is easiest to do for 'crazy' opinions coming from strangers.
Compare that with what can happen in real life. I get to know someone over the course of multiple interactions; all my political comments are kept to those I already know and trust. Eventually we get to a point we know and trust each other, and I venture an opinion, one they disagree with. They say "I disagree with that", and we discuss it. Why? Because we already know each other, value each other, trust each other, and are seeking legitimately to understand and/or sway one another, while also preserving the relationship.
The internet has essentially made it so the thoughts we used to reserve for those we knew in depth, are becoming aired to strangers, and responded to accordingly.
While it means that insular bubbles of thought (the rural family who doesn't know anyone who believes in gun control, say) are encountering people who disagree with them, those people are strangers, and there's no reason to give them any credence. If anything, it just makes one even more vociferous, because now you can preach to people who -aren't- in the choir.