[tptacek]

I think they might literally be the only one? Here's a quote I found (this time on Marginal Revolution):

It should be shocking, but it probably is not, that according to the Rule 606 reports mandated by the U.S. Securities and Exchange Commission, no major online broker, with the sole exception of Interactive Brokers, sent more than 5% of its orders to an organized exchange. More than 95% of their orders go to internalizers!

Later: Like, a minute later I read their Rule 606 report, and they do in fact get rebated for sending order flow to dark pools. They also get paid to take institutional orders and trade them against their customers.