The integration will let you choose Spotify or Apple Music from Messenger's built-in apps list, search for an artist or song and share the music straight into the chat itself, without opening any third-party apps at all. Once shared, the music can be played right in the Messenger chat, so the receiver of your message will not have to open Spotify or Apple Music either.
What wasn't made clear during the presentation was whether the music playback will be synchronized between chat participants, but we doubt this is the actual purpose of the feature. It is probably meant for users to have an easier time sharing music they discovered with their friends.
Also, no other music services were mentioned during the presentation. That being said, the integration of Spotify and Apple Music is based on Messenger's so-called Chat Extensions, which are open to third-party developers, so we wouldn't be surprised if other music streaming platforms take it upon themselves to develop Messenger extensions for their services.