That is a good sign, for sure. Maybe revise to 'Facebook will eat 20% of your lunch - your business model will need to account for that'. Products like this don't have a big moat other than their network.
Products that truly intend to sow community will be okay with losing 20% of the crowd which are indeed a group who will put forth lower effort, higher noise than signal contributions, and severely change your offering's requirements.
Think HN or lobste.rs vs....well, Facebook!