In my opinion, to be truly confident that your data will be forever exclusive to yourself and people you choose is to pay for the service, to keep it running in the long run.
Otherwise, how is the service going to survive? They have to monetize somehow down the road to pay the bills.
Charging money just by itself isn't enough to boost my confidence, charging money and reaching a publicly obvious critical mass of users combined would help but then you've still got a chicken and egg problem in creating that critical mass.
Good point. I think it helps to learn more about the people behind the app and their motivations.
I made Handpick (http://handpick.me) as a side project and have not taken any outside investment. Hence I have no investor pressure to obtain a critical mass of users in an insane period of time.
http://alvinlai.com/nanotate/