There are dozens of HFT players, all endlessly leap-frogging each other. Despite the fact that HFT is actually a relatively tiny market (the crumbs claimed by the HFT market account for at most a couple of billion a year), and is grossly overstated in significance.

In the broader market there are tens of thousands of major players, and millions of smaller participants. There is no network effect, and the market is so enormous that it doesn't concentrate.