How many people does the average person make contact with in a 5 year time span? Let's call this number X for hop 1.
How many people does each of the people in X then make contact with in the same 5 years? If we assume that on the whole people know ABOUT as many people as others, we can also call that number X for hop 2.
Now of course there's going to be overlap, out of X people somewhere between 0% and probably 70% might overlap between any one person of interest, hop 1 and hop 2.
Let's say that on average there's a 40% overlap. And let's call the Sweep(X) the number of people that get caught up in surveillance as a result of a single suspected terrorist.
Sweep(X) = 0.4 * X (first hop) x 0.4 * X (second hop)
Sweep(X) = 0.16 * X ^ 2
That's quadratic and as anyone who knows math, and especially people who know about Big-Oh notation, it's bad because Sweep(X) grows much faster than X does.
But how big is X? Well we can look to Dunbar's number to give us somewhere to start. Wikipedia says that it's between 100 and 250 but most people use 150 as a reasonable approximation. Of course that's STABLE relationships, not acquaintances, friends-of-friends, or anything like that. I think it's pretty reasonable to assume the 150 number and then assume that you probably make contact with one random person every week. Over 5 years, that's another 250. http://en.wikipedia.org/wiki/Dunbar's_number
So if we assume that X is 150 + 250 = 400 what do we get?
Sweep(400) = 0.16 * 400 * 400 = 25,600 let's just say 25k for short.
Now there are around 300 million people in the US. 300mm / 25k = 12000. That means that the NSA need only find 12,000 people (on average) to get all the call records for everyone in the US for the last 5 years.
It gets worse, though. What happens if you make contact with TWO random people every week? Then X = 150 + 500 = 650
Sweep(650) = 0.16 * 650 * 650 = 67,600. 300mm / 67k = 4477 people suspected of nefarious deeds in order to gain all call records.
As you can see this gets really out of hand really fast. And that's why I'm a bit upset that nobody's doing the math around the "two hops" argument because I assure you that the folks at the NSA aren't rolling over and giving up. This is something that SOUNDS reasonable but in actuality means little/no reform whatsoever.
EDIT: wikipedia link