Mommy Facebook groups are harmful for the same reason they're beneficial: they bring isolated people together. Motherhood is extremely difficult, especially at the beginning. People shame mothers for only wanting to talk about their babies, isolating them from friends and family. Often, they have a spouse who works outside the house, so they spend most of their waking hours alone with their kids. Anyone as lonely as that longs for a community that understands their unique struggles and doesn't shy away from discussing the ugly side of being a mom.
While that's all well and good, mom Facebook groups can isolate women uniquely. They are recruiting tactics for various fringe groups that can further bridge the gap between mothers and their real-life community. They are breeding grounds for mom-shaming, deserved and otherwise. The "scientific" information spread in these groups is suspicious at best and dangerous at worst. It's far better to look and laugh from afar than to subject yourself to the nonsense of your city's mom group. Trust me on this one.