I've told a couple of close friends and family who have been really supportive. If they are really close friends and you feel comfortable, I don't see why you couldn't. I write romance though, so it's a little bit of a different story.
I highly recommend not telling people what you write online. I made this mistake and got royally screwed because I thought I had a trusted friend on here and I didn't. They ended up leaving me a negative review on one of my books and tanked it right when it released, I was furious. Plus, now they write in my genre because they saw I was doing well. Pretty sure they were just blatantly using me for information. I'm now constantly paranoid they're going to leave more negative reviews on my stuff. All in all, it was my fault, I should have known better, but I'm just sharing my experience so you don't make the same mistake I did (which I doubt you would but anyway).
So, my two cents, if it's someone you care about and trust, it's sometimes nice to have a confidant. My friend has left me good reviews on a few of my books, which was nice. And my parents actually know I write romance and they have all sorts of fun questions..lol But I would never, ever, tell anyone online.