I have also been skeptical of the carefully worded releases, but looking at the PRISM slides more carefully just now, nothing on them necessarily indicates that the target companies actually know what is happening. Perhaps PRISM is based on partnering with backbone providers to suck data straight off the pipe, and the "Dates When PRISM Collection Began" refer to dates when they completed software to scope out information specifically destined for or leaving the services of each "provider".

This is just a possibility - I tend to believe the companies are simply lying. But it is possible.