It is too general of a question. The answer is that it depends on how they implemented it. I don't know how any of the cloud providers out there right now work, but I have read whitepapers on some cryptographically secure ones. The best designs use encrypted keyword search / private stream searching. Everything you upload to the cloud is encrypted client side, and you can retrieve it by searching for keywords, without the server being able to tell the keyword you are searching for or the encrypted file that you get from it. So that is pretty secure I think, although it still has some problems, unless files are padded they could possibly be fingerprinted based on their exact bitsize. I would in general suggest that you never store sensitive things on remote third party servers like that, but some of the whitepapers for cryptographically secure cloud storage look fairly solid to me.