(I believe Apple/Google/Amazon only provide discrete music downloads, rather than streaming services. Apologies if I had misconceptions about their service offerings)
That doesn't seem to me like they're small fish compared to twitter. given how much valuations fluctuate (Pandora was at $4 billion as recently as 2011), I think they're all in the same bin.
After all, Twitter's valuation is based on a less certain business model than the other providers, who have successfully implemented ads and premium accounts, and have paying customers.
(Pandora is an interesting case, since they've publicly said that they're not going to be profitable for another 5 years...)
Soundcloud is pretty big too.