I found this new and quite interesting version of the cloud storage report which talks about the market share of Dropbox, Google and Microsoft. Again not sure how representative it is but it seems like the author has some data to work with.
Maybe. This report seems to just take Dropbox Google Microsoft and Box into account. But is iCloud really that big when you talk about usage? They store tons of data but most of it might be just iOS backups which are activated by default for every user. Consider that usage or not.
I've heard of them before a couple times but frankly, I don't know how good the source is. Just considered it interesting and I'm a Dropbox only user as well.