According to a newly published report from Strategy Analytics, more of a survey than anything else, iCloud accounts for 27% of cloud use with Dropbox and Amazon trailing. However, I don't know if Apple should declare any sort of victory here.
The fact that Amazon is third over all with 15%, ahead of Google at 10%, suggests that a closed system like Apple and Amazon are more able to get their users to sign up for their own services.
If you want something amazing about this report, it's that Dropbox is in second place with 17%, ahead of both Amazon and Google. Dropbox has no support in terms of native OS. What it does have is an open strategy to allows its services to be used on any PC or mobile device with plenty of app support.
For iCloud, its use will be limited to Apple's ecosystem. That is its advantage as well as potential pitfall. Apple has to continue to innovate and expand its cloud offerings, an area where many consider to be its Achilles' heel.
Right now, music storage is the main function of these cloud services. However, all this can change once video streaming becomes more mainstream. Also, it depends on how fast video streaming is adopted by cloud services.
Another issue will be about the limited storage now provided by iCloud, Dropbox, and Amazon. 5 GB seems to be standard but looking for that to be increased as users become more comfortable with entrusting their files to giant server farms.