Earlier quoted context omitted.
You are right. What you say about Facebook is true, but Google's Android is open source, so there is no way they can plant a privacy-invading code and get away with that.
Just because it's open source doesn't mean it's safe. You have no control of what happens to that code before it gets installed on a phone. Samsung, whomever can and do modify the code -- those modifications aren't generally open source. Ruby on Rails is open source but that doesn't mean that all applications on rails are open source.
That will certainly change quite soon. In the earlier days, FOSS was not a concept that masses were aware of, but now is different. There is increasing competition in the smart-phone world and if one of the other manufacturers (say ASUS) makes their Android modifications open-source, they will see a drastic increase in Sales. To keep up with competition, Samsung, etc. will also have to do the same. In other words, competition will ensure the success of open source.