I believe the main reason is our way of talking. We are so sure we’re on the “right” side and acting in the public interest that we end up lecturing instead of trying to persuade. And no one takes kindly to that.
If we really want to do some good, we need a different approach, rooted in respect for our friends and their right to come to their own decisions. Speaking from the heart about why we feel the way we do — not why we are right — is key. It may seem paradoxical but, in my experience, this is the best way to get friends to take our arguments seriously and possibly, over time, change their minds.
For specific suggestion on how to talk to friend about environmental issues — and stir up interest on Facebook — see Becoming an Earth Ambassador on This Green Life.