How effective has mass surveillance been in preventing terror attacks since 9/11?

You are still being watched, and your data being collected, so be careful what you send and to where you send it too. Innocent messages sent out of the country can easily make you become a person of interest for no reason.

It is possible to have security, but not privacy, so although you believe your data may be secure, your privacy can be being invaded. You wouldn't know by who, so the best thing to do for now until privacy is better regulated is to be mindful of what you are doing on the internet.