By Michael Johnston

Last week the ETF Industry Association released its half-year batch of ETF data, providing an interesting snapshot of this fast-growing industry. It should come as no surprise that ETF assets continue to surge; the industry is quickly closing in on 1,500 products and $1.2 trillion in assets.

Looking a bit more closely at the stats for the ETF industry, however, reveals a number of interesting nuggets--below are ten facts about the pace of expansion that surprised us:

10. PIMCO's Year-Over-Year ETF Growth: 330%
Pimco was a relative latecomer to the ETF industry, but is wasting no time catching up to more established rivals. Thanks in large part to the extremely successful launch of an ETF version of its Total Return bond fund, Pimco's ETF assets have more than tripled during the past year. BOND has gathered more than $1.5 billion in assets since its March launch, and could be well on its way to becoming the largest fixed-income ETF.

9. ETN Growth: $3.7 Billion In Inflows
Some investors remain hesitant to use ETNs, preferring to avoid the credit risk that comes along with this structure. But the popularity of ETNs is clearly growing; already in 2012 more than $3.5 billion has flowed into exchange-traded notes, and the total now stands at about $16 billion.

8. Fewer $100M ETFs Than 2011
The ETF industry has always been top-heavy, but it was surprising to see that there were fewer ETFs with at least $100 million in assets at the end of June (521) than there were at the same point a year earlier (524). Part of that is no doubt attributable to the weak environment for stocks over that period.

7. Top Ten ETFs = 35% Of Industry
This stat illustrates the imbalance in the ETF industry quite nicely; the ten largest ETFs-SPY, GLD, VWO, EFA, EEM, QQQ, IVV, TIP, LQD, and VTI-make up more than a third of total assets.

6. Bond ETF Boom: Lineup Expands By 91%
At the midway point of 2010, there were only about 116 bond ETFs on the market. Now, there are 221 fixed income ETFs out there, many of which offer targeted exposure to international and domestic debt markets. The growth in assets has been equally impressive; bond ETF AUM has grown by 46% year-over-year and is up almost 87% compared to the same point two years ago.

5. Vanguard Doubles AUM In Two Years
Vanguard has been red hot in terms of attracting new assets; the low cost provider has led the way in most months for the past few years in terms of inflows. Vanguard's ETF AUM has more than doubled since June 2010, now topping $200 billion. Vanguard leads the way in 2012 with nearly $30 billion in new inflows since the start of the year.

4. Active ETFs Catching On
Active ETFs have been slowly but surely gaining steam, as illustrated by the impressive jump in assets for AdvisorShares. The provider of active ETFs now has assets of more than $450 million, indicating that investors are in fact embracing these vehicles.

3. Currency ETFs Losing Steam
One area of weakness has been the currency ETF space, as it seems that investors are losing interest in betting on exchange rate fluctuations or hedging exposure with ETFs. Currency ETFs saw almost $2 billion in net outflows through the first six months of the year, a significant difference from other asset classes.

2. LQD, VWO Off To The Races
The individual leaders in terms of year-to-date inflows are a bit of a surprise, especially given the rocky environment lately for international stocks. Vanguard's Emerging Markets ETF (VWO) led the way with $7.4 billion in new assets in the first half, followed by the iShares Investment Grade Corporate Bond Fund (LQD, which has hauled in almost $5 billion). Those two ETFs account for more than 15% of total ETF inflows.

1. Nine Issuers Rake In $1 Billion
Though the industry continues to be dominated by a handful of large players, several issuers enjoyed impressive growth in the first half. Nine companies, including Schwab, Van Eck, ALPS, and Wisdom Tree, took in $1 billion or more in new inflows in the first half of the year.

Michael Johnston is a a senior analyst at ETFdb. ETFdb offers a comprehensive and orignal ETF database and analytical consulting services for advisors and investors, as well as a free newsletter. Learn more about their services by visiting  Disclosure: the author had no positions in the securities named in this article at the time of writing.