Paris Terrorists Didn't Bother to Encrypt their Email, IMs, Smart phones, or Laptops & Still Weren't Detected

That hypothesis is not the one preferred by Hanlon's Razor: never attribute to malice that which is adequately explained by stupidity.
 
The more information you collect, the more reduction and analysis you need to do in order to make sense of it.  The signal-to-noise ratio inherent in collecting all communications on the Internet is so incredibly low that you would need a legion of analysts to put together a story that makes sense.  Even with megabuildings packed with server-quality computers, the bottleneck is still in acquiring employees that know what they are doing, and are willing to do that kind of work.
 
Compared with the private sector, the government pays peanuts and demands so much more.  About the only way they can get skilled programmer-analysts is to train them up from their own military and then move them to a defense contractor that can offer pay not tied down by the government employee pay schedules.  Even then, a Manning or a Snowden might start to feel slimy and blow their whistle.
 
I feel confident in saying that no, nobody detected it.  It was an attack with man-portable explosives and small arms on targets of no military or infrastructural value.  It fell outside the "defended area", so the initial automated screening algorithms never flagged it for human review, or it was never passed on to any group that could act on the intelligence.
 
They didn't ignore it because it suited their purposes to let it happen.  They ignored it because they don't actually care about that kind of thing in the first place.  Governments just don't care about protecting the servant classes from attack, except to the extent that it makes them appear weak.