privacy and facebook

I adopted my daughter when she was 3 years old from department of social service. She was the foster daughter of my mother in law. When she was free for adoption my husband and I adopted her. I also have another daughter by birth. My adopted daughter is now 16 years old and my other daughter is 17. My adopted daughter has a facebook and her birth mother has sent her several friend requests. I don't think she knows for sure that this is her daughter. We kept a version of my daughter's first name and I think she saw it on my mother in law's page as a friend. My mother in law did not understand to have tight security on her page. My adopted daughter happened to mention to her sister that she hates it when people she doesn't know try to befriend her. My older daughter got suspicious and mentioned it to me. I went on my adopted daughter's facebook and blocked this woman without telling my adopted daughter. I am adopted and I found my birth mother several years ago. But I had my own family, a house, a career and I knew who I was and felt comfortable in my life. I think being 16 is hard enough without having your birth mother walk into your life. I told my adopted daughter that I would help her look for her birth mother some day if that is what she wanted but I don't think she is ready for that kind of disruption in her life. I am ashamed that I blocked her birth mother on facebook without telling her. I was also scared to death. I was not ready for this. technology today makes it so easy to find people. I found my mother on an adoption search site. I am not sure if I should tell my adopted daughter about her birth mother trying to contact her.