PRISM got its name from the fact that the infamous "Room 641A" fiasco from the mid-2000s with AT&T and an NSA whistleblower leaking info about how prisms (literally, real prisms) were being used to beam-split the fiber optic feeds that make up a huge portion of the Internet dataflow backbone nowadays (aka trunks) and if you wanted to copy the data being transmitted over said fiber optic links the easiest way would be to just use a prism to split the beam (literally) and let one part continue on while collecting/copying/harvesting/mining the remaining side.
It's literally a perfect tap in most every respect with no real trace or tampering of the original bitstream that you could note to any degree at all.
I'm one of those 'tinfoil hat' people I suppose: been involved with data communications since the 1970s (yes I'm somewhat old these days) and I've always been well aware that this sort of thing - meaning the active data collection/mining/monitoring - is not only possible but relatively easy to do. In the past it was more precise and more targeted because even though it was easy to do, it wasn't easy to do on a large massive scale so in those days you would literally have to know exactly who, what, when, where, and why you were allowed to monitor or collect data on a given subject or target.
Nowadays, because computers are capable of monitoring so much data in real-time and finding those dreaded keywords or topics that set off the "red flags" to alert a human that something could be potentially worth more direct inspection, it's even easier.
I hate to say "I told you so..." (meaning I've been saying this stuff has been happening since the dawn of the telecommunications age and with it being primarily digital it has been in motion since the first router was connected to "the Internet" long ago) but but but..."I told you so... and so have many many other people over the years, some still with us, many long gone."