Basically Thomas Peterffy. I've read their reports a couple times which explains it in detail, but this image I just found on their site pretty much highlights it:
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Effectively IBG LLC is the broker that we all know. IBKR (the stock) is the "Public Stockholders" in the attached image. "Members of IBG Holdings LLC" is effectively Thomas Peterffy and a few associates. As you can see, the IBKR stock effectively only owns 17.8% of the brokerage itself by way of "Interactive Brokers Group, Inc", but the majority ownership and voting power are controlled through IBG Holdings LLC, which is not publicly traded.
So what does IBG LLC own? Does it own all of the brokerage firms like IB Canda, IB UK, IB Hong Kong and etc.? There is no lines extending from IBG LLC. It looks like it doesn't own anything. LOL
I think the Zuck has > 50% voting in FB.