Medicare is actually NOT a main source of referrals for LTACs; Medicare does not have a specific benefit for LTAC, and considers the skilled days to be the same as subacute rehab days. Sooo they would naturally prefer to pay for treatment at a much less expensive subacute than at an LTAC unless absolutely necessary.
This also means that if a person does end up in an LTAC on Medicare, they will typically run through their allowed days pretty quickly, considering that there is an extended average LOS.
In my experience, working in an LTAC for a few years, Medicaid and most importantly Medicaid-pending, and Charity are the most common payers. Commercial insurances are much less common, as they also consider subacute rehab to be preferable.