From The Guardian lawyers and investigators who were involved in the cases that Feinstein and Rogers cited to justify the data dragnet have cast serious doubt on their claims.
Lawyers and intelligence experts with direct knowledge of two intercepted terrorist plots that the Obama administration says confirm the value of the NSA's vast data-mining activities have questioned whether the surveillance sweeps played a significant role, if any, in foiling the attacks.
But court documents lodged in the US and UK, as well as interviews with involved parties, suggest that data-mining through Prism and other NSA programmes played a relatively minor role in the interception of the two plots. Conventional surveillance techniques, in both cases including old-fashioned tip-offs from intelligence services in Britain, appear to have initiated the investigations.
I strongly recommend reading the entire article. This is a convincing illustration of why we need open public debate on these programs. Such encroachments on civil liberties can only by justified substantial verified results. It requires a lot more than DiFi telling us to just trust her.