As of 2010, 50-70% of all stock trades were done by high frequency trading computer algorithms.
And many other asset classes are dominated by high frequency trading as well.
High-frequency trading distorts the markets. And see this, this and this. And it lets the big banks peek at what the real traders are buying and selling, and then trade on the insider information. See this, this, this, and this.
Morgan Stanley has just shown (via the Financial Times) that the percentage of high frequency trading in the stock market has skyrocketed to 84%:
Trading by “real” investors is taking up the smallest share of US stock market volumes [since Morgan Stanley started keeping track 10 years ago.]
The findings highlight how US trading activity is increasingly being fuelled by fast turnover of shares by independent firms and the market-making desks of brokerages, many using high-frequency trading engines. [actually all of the market-making desks are using it.]