This reply isn't meant AT ALL to be political (I'm a big fan of capitalism, probably fall closer to the "rich" side than the "poor" side and don't begrudge anyone earning and keeping anything), so don't take it that way and let's try not to slip down a political rathole...
With that said, I think the statistic presented is completely meaningless...
This is one of those situations where talking about "averages" is very misleading. While the average median networth may have dropped considerably over the past few years, I'd be willing to bet that the average networth has not.
If you're rich, you probably lost a considerable amount of networth back in 2007-2008. That's because you likely owned a disproportionate amount of capital assets that devalued quickly and steeply doing that period. Certainly, most people in this country lost a lot during that time, but among the "average Joe," there were fewer capital assets owned and therefore less of a proportionate drop (though if your sole asset was your primary residence, you probably lost a lot as well).
Then, during the period from 2009-2012, if you were rich, you likely had some opportunities that were pretty unique in an historical sense -- real estate was as discounted as its ever been, taxes were low, fear was rampant, government policy favored them, etc. Certainly, business owners were being hit hard, but if they had cash reserves, they had an opportunity to build networth through acquisition of capital assets outside their business. Of course, if you're rich, you're more likely to be intelligent about how to acquire wealth/cash, so there's that advantage again (and again, that's what capitalism is all about, those that have the resources and know-how and work ethic will get ahead -- not a bad thing).
Unfortunately, during this same period, those with little cash reserves and no entrepreneurial spirit had essentially no opportunity to get ahead and the world around them in economic turmoil. Employment was down, real estate (for many, the only source of capital assets) was getting crushed, and credit was tight. If you were poor or middle class from 2009-2012, you probably lost a lot more than 40% of your networth.
So, here we are now. The rich have for-the-most-part recovered or positioned themselves to recover from their losses from 2007-2008 (if they were smart), the ultra-rich have gotten ultra-richer or positioned themselves to get ultra-richer (again, if they were smart), and the poor (and even many middle-class) are now in a holding pattern waiting to see what happens around them.
The 40% drop in median networth may be a correct figure, but it's certainly not representative of what happened on the ends of the spectrum -- many lost everything and many increased their wealth considerably (or have positioned themselves to do so).
Btw, I'm guessing that many here on BP were smart enough to at least tread water for the past few years, or if they did lose a fair amount, they were smart enough to figure out how to recover or position themselves to recover.
Now feel free to proceed to beat me up... :)