I have a myspace page and have it set to private so that only people that I approve as friends can see anything on my page. Maybe you could do that so that biomom can't read your stuff.
I had a similar problem with my dh ex with a page I had on a website called babyfit. She found it somehow and was reading all my stuff, so I cancelled the account. BUT for my myspace page I made it private so that even if she finds me, she can only see my name and pic and can't read anything on my page or blog.
You are only human and have the right to talk to friends. What you said may not have been right, but it wasn't right for her to read it and repeat it. But from reading your other post aout what you are going through with her, it doesn't surprise me. Believe me I share your grief/pain in the situation. I know EXACLTY how you feel. If you don't let it out and talk to someone it will build up. Does your hubby know about what happened? I am guessing he does and if he is supportive of you, I suggest talking to him about it first and see what he thinks. That doesn't mean you can't do what you feel is right in the end, but you can at least get his imput. How old is your stepdaughter? Is she old enough to understand that sometimes grown ups need to talk to other grown ups about things? Maybe you could explain that to her. Does she seem to be upset about what happened? Has it changed your relationship with her? If it does seem to have bothered her or she isn't as open with you as she was before I would approach her and just try to explain that you needed to get things off your chest with other adults. I would also explain that it wasn't appropriate for her biomom to repeat to her what was said.
I know I haven't been much help. But again, I suggest that you set your myspace page to private and do not approve her as a friend, to prevent this in the future.
feel free to email me if you want. I feel for you!
Hugs and blessings! K.