If the data or encryption key is out of your possession, you must assume it is public. If you want to secure your data, it must be encrypted before it leaves your computer. And if you want to trust your computer, you can't use a proprietary OS.
Most people don't need that level of security... some convenience is worth the likely loss of privacy (to a point). I'm not going to worry about getting my cousin to use PGP in order to email about our family reunion. But if you are concerned about privacy, you have either already eliminated cloud services from your daily workflow or you are an idiot.