Pandora is a personalized Internet radio that can be used free of charge. Though it is free, you can pay to get added features. The service is made personal by creating different radio stations for the owner. The owner will just pick a song or artist they enjoy and songs/artists that are similar to that one will make up the station. Since Pandora is free to users, they use commercials as a way to fund Pandora. PR professionals can utilize these commercial times to promote their client and they may choose to promote certain clients with certain stations.
Spotify is similar to Pandora in that it is a music playing service. It can be used on most electronics and it is free as well. Just like with Pandora, the catch with it being free is that you have to listen to advertisements. Spotify is very similar to Pandora, but it doesn’t focus on the stations as much as it does it’s wide music selection. I would recommend that PR professionals approach Spotify as they would Pandora.
Slacker is also very closely related to Spotify and Pandora; it is a radio station service as well. It also provides the free option that contains advertisements or you can pay to get rid of the advertisements. It appears that Slacker does more videos though compared to its competitors, though the other services are going that direction as well.
I personally do not see much of a difference between these 3 services; they pretty much all offer the same product. From a PR standpoint, professionals may also want to look into utilizing any social media accounts that the services have.