Yes, but that's very different from what the banks are doing.
It's certainly a possibility that an exchange uses, say 50%, of the BTC it has, to invest around at its own risk, doing things like selling those BTC and invest the cash into stocks, but that's no money creation. It's more like if I lend you my car to do your shopping, you use it to go racing.
technically speaking, the BTC exchange don't have the rule of central bank (real money creation, namely BTC mining), but the rule of commercial bank.
But they can operate EXACTLY as a commercial bank (commercial bank are 99% of banks)
from
http://en.wikipedia.org/wiki/Fractional-reserve_banking:
Money creation process
Main article: Money creation
There are two types of money in a fractional-reserve banking system operating with a central bank:[15][16][17]
Central bank money: money created or adopted by the central bank regardless of its form –
precious metals, commodity certificates, banknotes, coins, electronic money loaned to commercial banks,
or anything else the central bank chooses as its form of money
Commercial bank money: demand deposits in the commercial banking system;
sometimes referred to as "chequebook money"
When a deposit of central bank money is made at a commercial bank, the central bank money
is removed from circulation and added to the commercial banks' reserves
(it is no longer counted as part of M1 money supply). Simultaneously,
an equal amount of new commercial bank money is created in the form of bank deposits.
When a loan is made by the commercial bank (which keeps only a fraction of the central bank money as reserves),
using the central bank money from the commercial bank's reserves, the m1 money supply expands by the size of
the loan.[2] This process is called "deposit multiplication".
