Virtu Financial is a public company so you can read quite a bit of detail about the business in their SEC filings ... from the S-1:
"Historically, our competition has been registered market making firms ranging from sole proprietors with very limited resources to large, integrated broker-dealers. Today, a range of market participants may compete with us for revenues generated by market making activities across one or more asset classes and geographies, including large broker-dealers, such as Bank of America Merrill Lynch, Citigroup, Goldman, Sachs & Co., Morgan Stanley and UBS, and other participants, such as Citadel, DRW Holdings, Hudson River Trading, IMC, KCG Holdings, Optiver, Susquehanna, Timber Hill and Wolverine Trading. Some of our competitors in market making are larger than we are and have more captive order flow [emphasis mine] in certain assets. We believe that the high cost of developing a competitive technological framework is a significant barrier to entry by new market participants.
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As a self-clearing member of the Depository Trust Company ("DTC"), we avoid paying clearing fees to third parties in our U.S. equities market making business. In addition, because of our significant scale, we are able to obtain competitive pricing for trade processing functions and other costs that we do not internalize. Our significant volumes frequently place us in the lowest cost tiers of brokerage, clearing and exchange fees for venues that provide tiered pricing structures. Our low-cost structure allows us to maintain a marginal cost per trade that we believe is favorable compared to our competitors. Our scale is further demonstrated by our headcount — as of June 30, 2015, we had only 148 employees.
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Rebates consist of volume discounts, credits or payments received from exchanges or other market places related to the placement and/or removal of liquidity from the order flow in the marketplace. Rebates are recorded on an accrual basis and included net within brokerage, exchange and clearance fees in the accompanying condensed consolidated statements of comprehensive income."