Icebergs are when someone is trying to do a much larger trade, piecewise. This is what my group at MS facilitated (automated algorithmic execution of order flow.)
Hidden quantities are an order type that some markets have that allow you to hide the total amount you place in an order, more or less.
As luck would have it, one of my best guys is a former MM -ish trader on the CBOE.
For whatever it's worth: total agreement with you that GS isn't acting as a MM (after an earful about delta and position risk and volatility risk and long strangles and gahhhhh). He says "GS is a BD, MM's can't trade for clients" and a bunch of other stuff.
You are 100% right. GS is not a market maker. "Doing trades that help provide liquidity to Facebook instruments that somehow down the line helps build a market for Facebook" is in no way the same thing as "being a market maker". People should stop saying "market maker" when they mean "market helper" or something else.
On the other hand, had no clue what a hidden quantity was; neither did my exchange engineer friend. So I feel a little less dumb. He knew what an iceberg order was. Also, note that there's at least one exchange that doesn't draw a distinction.
Hidden quantities are an order type that some markets have that allow you to hide the total amount you place in an order, more or less.
Client-side versus server-side, if you will...