Except that the Fed is not keeping the federal funds rate low. The Fed has raised the rate twice already this year as well as once at the end of 2016. They have also signaled that another increase is coming in December. See:
Yes but I was responding to the assertion that "The federal reserve is keeping the federal funds rate low ...", when that hasn't been the policy for almost a year now.
So the current Fed policy is not in fact indirectly subsidizing ride sharing.
It has raised the rates from extremely low to very low, which is still keeping rates low.
Now, you can argue that that is in part because a rapid increase to non-low rates would be disruptive given the status quo ante, but that's a justification for keeping rates low, not a rebuttal to the claim that that is what the Fed is doing.