Investors have flocked to exchange-traded funds and exchange-traded notes through the first eight months of the year.

Year-to-date through August, net cash inflows were $73.2 billion, a 54% increase over the same period in 2010, according to data from National Stock Exchange, Inc. ETFs and ETNs posted net cash inflows of $2.03 billion last month alone.

Tom Graves, an equity analyst at Standard & Poor’s, called the year-to-date number “quite impressive.”

Even the $2 billion of August inflows, which is lower than the monthly average for the year, is strong, considering the volatility in the equity markets, he added.

Assets in U.S. listed ETFs and ETNs totaled approximately $1.064 trillion at the end of August, up 31% from a year earlier, according to National Stock Exchange. 

ETF/ETN national trading volume last month totaled approximately $2.99 trillion, representing almost 37% of all U.S. equity trading volume. 

Meanwhile, exchange-traded products have continued to proliferate. As of the end of August 2011, there were 1,301 products listed, compared with 1,046 at the same time last year. 

“The number of ETF securities is, to me, a further indication that we’re going to see a growing amount of products, not only from existing ETF providers but from newer ones also,” Graves said.

Blackrock remains the ETF heavyweight; it has 214 products and $444.6 billion assets. Those assets were down from $472.9 in July, however. State Street Global Advisors and Vanguard are second and third, respectively, in terms of total assets.