...In the end after listening to and reading all the evidence I presented her, she resolved to accept the ridiculous youtube videos saying seeing was believing and we never went to the moon...
In the center of my avatar image, the little white speck casting a shadow is the bottom half of the Apollo 11 lunar module, photographed from lunar orbit in 2009. Of course, "seeing is believing" generally applies only to things that the seer already believes, so this won't convince anyone.
Back to the original topic, I've been heavily involved in a few online forums for many years, and the "echo chamber" effect is very real. Even if a community starts out with a range of opinions, it frequently happens that a small group of true believers will eventually annoy the heck out of everyone else, and drive them off. Forum moderators can encourage or discourage this, but never have complete control.
I don't think Facebook has improved the situation any -- it just makes it even easier to shut out ideas and people that you disagree with.
I find this discouraging. On the one hand, the Internet has become an amazing way for people to find friends and to learn things, with no artificial limitations imposed by physical distance. And on the other hand, so many of us use it solely to find clones of ourselves and echoes of what we already think.