I've been tracking volume on NYSE since 2007. This year, it's the lowest year since I don't know how long. It's barely keeping it's head above 1 billion shares per day, where as since 2007 at this point it should be trading over 1.5 billion. The volume has dropped by 33%. Where did this volume go, and what does that mean? Raging bull market on dropping volume... it's can't be a good sign, could it?
Volume has definitely been on the lighter end, but I think that the actual number recorded publicly is smaller than in reality... over the past two years, the use and accessibility of dark pools has increased. I'm not sure, but I don't think those trades are included in total NYSE/NASDAQ volume. With that being said, I think the NASDAQ is trying to get a system where all dark pools report their volume publicly.