From my understanding BTB is doing quite alright during peak play hours, but it can really struggle during the trough periods.
Just so you know Team Slayer (within the Ranked environment) is suppose to be getting several of the community slayer maps added to it, so maybe that can help entice your friends to try that playlist out. And it's worth mentioning that the matchmaker isn't really doing anything much different these days when searching within the Ranked Arena environment verse the Social Arena environment. The biggest difference is the time limited JiP (in Social) verse no JiP (in Ranked). Just ignore the visible ranks and focus on the match outcomes + gameplay.