Docs was google's only docs service, and subsequent acquisitions have been squashed into it (e.g. etherpad). Maps is google's only maps service. Blogger was more-or-less google's only blog service until recently, and I wouldn't count on it surviving much longer in the age of google+. Youtube happened the other way around, in that google killed google video and merged it into youtube.

Is play music more established than songza? I think yes, and google is unlikely to maintain both of them in parallel.