Bitfinex is pretty big.  The 24 hour volume is 32k BTC and 59k LTC.  

So it could be almost as big as Bitstamp (which gets a lot of its trades, possibly as much as a third, from BFX).

US lending is growing rapidly.  Several months ago it was around 1 million - now $2.7 million.

It isn't running out of money. US lending rates haven't risen dramatically over time (they are less now than during the crazy $266 bubble).

Unlike Bitcoinica, which limited lending rates to 20%, BFX has higher rates (typically 30-50% APR, or 100%+ in a crazy bubble) and thus more supply.