Correction to Airbnb example: (it is 3% to the host and between 6 and 12% to the renter.
If the 'supply side' gets 100% payment, there is no incentive for 'supply' to encourage going around the service especially since they receive benefits of secured bookings and payments.

With freemium like Craigslist and Kijiji there are some advantages with a large number of users. However, the quality of supply (bare bones listings), responses (people not responding or not meeting when they say they will) can easily be over 50% which hurts customer experience.