Proof. Overwhelming proof. And all of the media reacting on this at once. There have been a few stories about spying before but at best each site ran one story about it and that was it. The next day everyone moved on.

I think both the brilliance of Greenwald/Guardian to launch a new story every few days and the stupidity of the US government when abusing its power to stop the press or Snowden, is what kept this for so long, and hopefully will continue to happen.