Depending on the state, I believe it can be. Many states have "stalking" laws preventing just the sort of behavior Facebook engages in as part of their business

Depending on the state, I believe it can be. Many states have "stalking" laws preventing just the sort of behavior Facebook engages in as part of their business plan. In most (or all?) states, PIs are licensed and also have limitations on what they're allowed to do. There is currently no license for Facebook, nor any limits that they recognize.