"Another reason for this latest vacation rental twist is that it’s hard for the city to prove illegal use."

Agreed.  First the city would have to find someone who could read.  Then that person would have to figure out what "obscure" (all over the internet) social media site operates in such a fashion to advertise rentals.   THEN they'd have to pick up the phone to call the advertiser to get the address.  Walk ALLLLLLL the way down to the parking lot to get into a car and drive TO the property.  Knock on the door and ask for ID from whomever answers to be shown an out of state ID.  ASK that person what their arrangement is and from whom.  And then...and then.... START the paperwork.   Almost resembles a day's work.