Do we really "know" people?

I'm 45 and I have just realized that we don't really "know" people. What we know is through our own observations and from what they divulge. Recently I found out that someone who I thought I knew I really don't know. I feel like a tool. I always want to believe the best in people. I want to believe what they tell me. If I have taken my time and have allowed someone "in" I expect at least honestly. Not one sided bull shit. You know, I don't know if i'm getting across what I'm trying to say or not. If I have decided that we "click" and are friends and then years later discover you were a fake person and I have shared intimate details of my life it hurts me. Hurts me deep. If friends can't share "real truths" no matter how ugly than the one who isn't sharing is lying in omission.

I have a friend who has done just that. Let me know parts and pieces and never the real truth and now I'm stuck wondering if I should confront or back away and consider it a lesson learned and move along toward the next thing.?? Any suggestions?

I feel like an investment in each other is what deepens friendships and allows them to flourish. I'm so confused.