You and your husband are well-advised to monitor and talk to your step-daughter's mom about the child's online presence but I do not think it rises to the level of neglect. It is certainly not worth the hostility that would be caused between your husband and his daughter's mother to report this to anyone.
If your husband is unsuccessful in getting his daughter's mother to appropriately limit and monitor the child's access to Facebook and the Internet, he might want to talk to a family law attorney about filing for modification of the mother's parenting plan in order to impose reasonable limitations.
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I absolutely agree with Attorney Rich. You might also consider talking to your daughter about parameters for safe social media usage. Here is a great link on social media contracts for families: