Spotify has a $3 billion valuation as of last November. Pandora's value seems more in flux, but is probably somewhere in the 1-2 billion range.
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.
