Because here's the thing: most adoptees are going to have a desire to reach out to the biological family at some point. It is natural for adopted children to desire a connection to their roots and sooner or later they are likely to seek that connection ... with or without the adoptive parents' support. If you are an adoptive parent, wouldn't you rather be involved?
Open adoption allows the connection to biological family to occur in a safe, supervised manner, with the involvement of the adoptive parents. It can help to demystify the birth family, rather than positioning them as the forbidden fruit. An adopted teen won't need to sneak away to try to meet up with her birth mother if that birth mother is already a regular part of her life.
Yes, you should monitor your child's online activity and teach about internet safety, but as your child gets older, he or she will have opportunities to access the internet outside of your home and it will become increasingly difficult to monitor all social-networking activity. Keeping the lines of communication open between adoptees and adoptive parents, and, when possible, between adoptive parents and birth parents, can also be an important part of creating safety.
Being "Emotionally Open" to First Families
Let's Get Real: Embracing Duality in Adoptive Families
Are You an Attuned Adoptive Parent?