There are approximately 92 million ETH outstanding. It's much harder to measure market liquidity on the various exchanges and over-the-counter (OTC) markets.

With hundreds of ICOs popping up, it's difficult to say what impact they have on the market. Bancor raised approximately 400,000 ETH in their recent ICO, or 0.4% of all ETH outstanding, and the BAT sale raised 156,250 ETH. Prior to the attack, the DAO had attracted over 11 million ETH, or nearly 14% of all ETH outstanding at the time.

The fun thing is indeed that neither the Ethereum optimists nor the sceptics have concrete numbers to back up their statements.

The skepticism is healthy, the optimism interesting, the discussion unfortunately remains fairly immature, on both sides. I hope we'll get more academic and professional insight soon.