Helsinki, Finland had what amounted to a crowdsourced version of this that allowed reporting of any public safety issue: http://www.lahivinkki.com
The core problem with any service like this is twofold: you first need to get enough volume to make it sustainable, and then you need a lot of human moderation to clean up the junk.
I think I met half the addicts in Helsiki one night while jet lagged: The taxi diver took me to the wrong hotel - a Soviet-inspired hulk on the plaza with the bus terminal - after a meeting at Nokia. I woke up at 3am. Hadn't eaten all day. There was a 24 hour McDonalds next door. Yeah, that's who's there at 3am. The whole scene was about as candy-ass as the kleinmotorrad bikers in Vienna. Not exactly a no-go zone.
I see from the map that it's pretty likely that there was a good deal of commerce in other than Big Macs on that night.
The core problem with any service like this is twofold: you first need to get enough volume to make it sustainable, and then you need a lot of human moderation to clean up the junk.