The reason an open source platform is better is because, if you have strong concerns, you can get the source code and host it yourself. If you host it yourself, you have full control over the data, any data mining activities, etc. Open APIs such as Facebook's Dev API provide another mechanism for tracking use patterns and user behavior, and this information can subsequently be sold.

Privacy is only as good as the EULA, the privacy policy, and, at the end of the day, the amount of trust you have that a corporation or organization will tell you the truth.

Cheers,

Bill