In the Q4 conference call Peterffy talked about market shares. He mentioned them as 25% of proprietary trading groups, 15% of financially sophisticad individuals, 0.2% of hedge funds, <0.1% of RIAs. 1% hedge fund market share would be roughly $250m in commissions only, 1% of RIAs likely a nice sum too.
Currently the e-broker is doing about $1b revenues. I have no idea how big of a share IB could realistically capture of these markets over time. But if one believes that their technological advantage will enable them to win share over time across the board, then it's not hard to see how they could continue growing fast. As an example, getting a combined 5% share of HFs and RIAs would +2x sales.