PokeVision is a map tool used for tracking wild Pokemon around of you. It was developed by a third-party, and has been getting popular since the service is launched. However, now you cannot use it. PokeVision, PokeWhere, and other tools shut down suddenly.
Once visiting PokeVision page, the message 'Our service are currently unavailable' is displayed. The developer of the tool posted tweet about that problem as the following.
On the Forbes' interview to John Hanke, Niantech CEO, he addressed the map tools for tracking wild Pokemon. He does not want players to use such tracking tools and he argues developers of those tools violate contract.
F: How do you feel about Poké Radar and things that tap into the code and show where Pokémon are spawning?
We have priorities right now but they might find in the future that those things may not work. People are only hurting themselves because it takes some fun out of the game. People are hacking around trying to take data out of our system and that’s against our terms of service.