Recently I had a vaguely friendish person on facebook post a "question", the kind of leading BS question where you already know the answer and you're trying to inculcate your ideas in others, in regards to water fluoridation. It was something along the lines of "Does anyone know if this is true?" and then a link to a conspiracy site claiming that "government" is poisoning people by "dumping toxic waste" into our water supply. It had all the conspiracy bullshit flags...fringe sources, misunderstanding of the topic, misrepresentation of science, implications of shilling for "big something something", in this case, "big chemical". I responded briefly, and politely, that water fluoridation is no big deal, and often involves removing fluoride that occurs naturally. I provided source links to scientific sites.
The friendish person replied with, in hindsight, predictable vitriol, claiming I was mocking him, and that I was ignoring the evidence. He then went on a nearly page-long rant about how scientists are all in the pocket of industry and using "toxic waste" to keep the sheeple placid and accepting, and that I was indeed a sheep myself for believing their lies. Of course I at that point had realized my mistake, and responded very briefly that I was simply pointing out facts, never attacked him personally (like he was doing right then), and referenced the science again. Then I unfollowed the post. Shortly after that he began messaging me angrily, accusing me of everything under the sun, after two of which I blocked him. I won't accept personal attacks on FB or anywhere else.
I've got friends who believe all sorts of screwy stuff, and in general I just let it go. I'm not the thought police, and I'll generally only respond to things I perceive as harmful to the humanity, and even then sparingly. As Mr. Anti-fluoride proved, most people hold these crackpot beliefs dear to their hearts, and will defend them with vigor, despite their inherent stupidity. Anti-fluoridators, anti-vaxers, pro-"alternate medicine" people, and a list of others are pretty much all I'll respond to, and for very good reasons. A lot of those stances actually kill people (anti-fluoridation people aren't responsible for death generally, just making life a little worse for poor people). I'm not, however, a genius who somehow knows everything about everything, and don't intend to browbeat people over anything that doesn't violate my personal space. I'll try to engage people gently on some subjects, and sometimes it pays off.
One such time a friend posted something about homeopathy. Homeopathy is straight-up garbage and quackery, and many people don't actually know what it is...which is charging outrageous prices for completely pure water that has no clinical effect other than slightly greater hydration. I posted something along the lines of "Hey [friend], homeopathy doesn't really do anything, don't waste your money", and got a slightly irritated response. I figured that he didn't know what it was, and posted links explaining homeopathy. Indeed, he was thinking more of herbal remedies and naturopathy, which is completely separate, and responded with thanks. So a gentle engagement gave an opportunity to spread a little knowledge, and was well-received.
Most people I know will respond to logic and evidence. The thing I've learned which, looking back to my own more vitriolic past seems now obvious, is that not many people like to be insulted or bullied. While I've had to cut ties with some folks over various personal beliefs in the past, such as the Prop 8 fight here in California, and anti-vaccination stances, I'm generally more easy-going these days.
Our time is so brief.