I used to trust about EVERYBODY when I was younger... and it most definitely wasn't good. I've become better now, although most of the time I don't see what's wrong with giving people a chance for something and end up regretting it. I used to have a 'friend' who stole from me, talked with everybody else behind my back, used me for many things and lied to me. I suspected some things, but it wasn't until another of my friends told me that the so-called 'friend' had really been manipulating me that I decided to seperate myself from her. I really don't regret that. My other friend who I have mentioned is the one I trust most out of my RL friends, although she makes fun of me sometimes. But she's better than about anybody else.
My best friend, who I trust with anything, is in fact someone I met online. People online are generally less likely to secretly form against you, and that is something I appreciate.