Doesn't have to be a govt conspiracy, facebook will sell you IDs->IP address. It's literally their business model, answering the question "Who bought stuff where, or when do people want to buy stuff?". "where" and "when" can be derived from a time-series of IPs, for example to know if you observed something on your phone over wifi (implying you live with others, so network-based ads would be most effective) or cellular (implying you're alone rn + targeted ads are better).