According to Strategy Analytics, via Engadget, more people say they use iCloud than any other cloud service. This is most likely due to Apple pushing iCloud as a part of iOS.
Still, given all the hype around Dropbox and Google Drive, we thought they would be more popular. Strategy Analytics says awareness of cloud storage is still pretty low overall.
Music is the primary thing people are using with cloud storage services.
Source Business Insider March 2013