Today was pretty low - 1.7B shares in the S&P, but Friday's drop had the highest volume in a couple of years - almost 5B shares.
A couple of things to keep in mind re volume: today was a short day, and many traders also decided not to "show up" but to take a very long weekend.
Also, volume (+direction) as a technical indicator in and of itself doesn't mean a lot — Santa Claus rally usually takes place on a light volume... today it was a Santa Claus crash.
More important is a sustained number of 52-wk Highs and Lows and A/D (Advancing issues vs Declining issues) — e.g., today was 69 new highs vs 3151 new lows (!) and 1667 advancing vs 5531 declining. 771 above 200-day Simple Moving Average vs 6753 below 200 SMA. In other words, the breadth of the market is downright awful.
VIX was up almost 20% today and is now relatively high at 36.
Of course, it's always darkest before the dawn, and volatility is great for trading and there are always individual opportunities in issues or industries or sectors... but at this stage it seems it's way too early to invest long term in "the market" itself — the future earnings numbers are coming down pretty fast relative to current prices and economic activity is slowing down sharply around the world and about 45% of earnings at S&P 500 companies are from overseas.
There are still huge amounts of government, consumer and enterprise financing debt which is not spent most productively, to put it mildly (e.g., stock buybacks at the peak prices and ratios and expensive leveraged M&A and LBOs, like in "good ole' times") while the output productivity has slowed to a crawl.
FD (Full Disclosure): This is not an investment advice! My non-core portfolio is net short.