Yeah, the lack of liquidity can certainly be frustrating. It's such an amazing platform with minescule fees, I don't know why more people don't use it. Maybe it's a chicken/egg problem or maybe just a marketing one.
The BitShares/bitUSD is fairly active, but still nothing like you see on other exchanges. I've thought about setting up a market making bot to increase liquidity, but I'd probably just lose a bunch trying to figure it out.
I still have a hard time wrapping my head about where are the BTC and other crypto held.
Yeah, I get a bit confused by that also. Googling around a bit, I found this Steemit post helpful. From what I understand (which isn't much), it's similar to any other exchange in that you're trusting whoever runs that asset to play fair. If they go belly up, you may be out of luck, but the smart contracts of bitUSD and bitBTC can provide some security as long as the value of BitShares doesn't go through the floor.