The Fed Flow of Funds provides a nice proxy for outstanding Commercial Real Estate Debt. They are imperfect, though, it should be noted, and their estimate on CMBS tends to be a little blurred from reality because they only sample a handful of deals from each vintage (but its pretty close).
In the first chart, it's shows the total CRE mortgage market in blue, and then has the CMBS portion of the market overlayed. Sorry its a bit fuzzy.
Obviously the whole market has dropped off because originations have slowed, etc. and CMBS dropped off more than the market, also expected.
In the second chart, it breaks the total ($3.5 trillion-ish) out by MF and Non-MF. It's hard to see, and not because of my fuzzy chart, because it's soooo tiny of a move, but MF originations are actually up to $914 billion at the end of Q2 2009, from $905 billion at YE 2008, and $843 at YE 2007. A lot of that is government based lending, but also significant increases from REITs and Commercial Banks, while Thrifts and Insurance Companies have given up market share.
Outside of Multifamily, the lenders that increased their market more than just a few percentage points include the Government, well, and that's it. Commercial Banks (surprisingly to me) actually increased their market share by 8% since 2007, but most of the increase occurred in the first two quarters of 2008 and remained flat since. Specialty Finance, and REITs have both increased slightly. Most other lenders were flat or down substantially over the last 6 quarters.