That's arguable. I never had a chance to estimate this in earnest, but I have the hypothesis that the majority of HFT transactions are with one another. Sure, they collectively provide liquidity to non-HFT investors (because the order flow has to come from somewhere), but I also had the hypothesis that if the clearing houses (CME/NYSE/etc.) could improve their matching engines and/or consolidate disparate exchanges, that might solve the liquidity problem without HFTs.