Sorry @balt I don't have the article at hand. It was a while ago...

Sorry @balt I don't have the article at hand. It was a while ago so I suspect that figure has increased.

I also can't remember whether the 67% was by trades or volume. Maybe a bit of googling would bring it up - or a more recent figure.

Here's the first thing I found now:
Taken in the widest sense, algorithms are responsible for the vast majority of activity on modern stock markets. Apart from the “mum and dad” investors, whose transactions account for about 15 to 20% of Australian share trades, almost every trade on the stock markets is initiated or managed by an algorithm.
http://theconversation.com/explainer-the-good-the-bad-and-the-ugly-of-algorithmic-trading-68477