Halloween offered a nice treat for the exchange-traded fund industry.

According toState Street Global Advisors, ETFs saw assets up by 21.5% since last year as of the last day of October, attracting over $133 billion of inflows over the year. That's up from $99.8 billion as of October 2011.

October alone contributed significantly to these results. Investors added $2.7 billion more to ETFs than they withdraw. Fixed income attracted $4.9 billion in inflows while emerging markets ETFs drew $3.5 billion. Large-cap ETFs, however, saw outflows of $9.1 billion.

Dave Mazza, head of ETF investment strategy for the Americas, said 2012 represents one of the strongest years of ETF flows in recent times, crediting the efficiency, transparency, flexibility and precision of ETFs as major drivers for ETF growth. Those same factors should continue to motivate growth going forward into 2013, he said.

"Investors continue to seek ETFs to seek access to broad and narrow markets across multiple components of client bases — retail, intermediary or institutional," Mazza said. "This has been a market that has been driven tremendously by uncertainty, though it hasn't been as volatile as in recent years. There are a lot of question marks in investors' minds, and the transparency and cost effectiveness of ETFs resonate with investors."

Investors in particular have been attracted to fixed income, which saw nearly $5 billion in inflows in October and over $46 billion year-to-date, Mazza said. Equity as well has attracted investors particularly in ETFs focused on dividends, representing a move away from capitalization-weighted, traditional benchmarks and a greater focus on current income.

"Investors are not just yield-seeking," he concluded.