The exchange-traded funds industry continues to boom, as assets under management topped a record $1.29 trillion as of the end of the third quarter of this year, according to theETF Snapshot: September 2012, fromState Street Global Advisors.

Total assets have climbed 22.7% year-to-date through the first three-quarters of 2012, and there were 1,263 ETFs operating in the industry, the report noted.

In September, investors added net assets of $37 billion to ETFs than they withdrew, with emerging markets ETFs and large-cap ETFs attracting $4 billion and $10 billion of inflows, respectively. (For large-cap ETFs, this was a reversal from August, when those ETFs had outflows of $4.4 billion.)

Through the end of September, ETFs have attracted more than $130 billion of inflows—a stunning 68% increase from $77.2 billion the sector showed in year-to-date inflows as of the end of September 2011.

The report continued to show heavy concentration in the market for ETF managers, with the top three—BlackRock,State StreetandVanguard—accounting for 83% of the ETF market.

The three largest ETFs as of the end of September wereSPDRs S&P 500(SPY) with $118.3 billion in assets;SPDRs Gold Shares(GLD) with $75.3 billion; andVanguard Emerging Markets(VWO) with $57.2 billion. Combined, the assets in three largest ETFs accounted for about one-fifth of the entire market, according to the report.

A spokesperson at State Street Global Advisors did not return requests for comment.