No, unless they're Russian, they're not free falling. They have greater potential energy. And also increased traction, increased rolling resistance, and increased losses in wheel bearings and drive components due to friction.
Terminal velocity can still apply when cyclists are going downhill. Essentially it's the speed that a cyclist will reach when the gravitational force is equal to rolling resistance (roughly proportional to speed) plus the air resistance (roughly proportional to the square of speed) assuming that they're just free-wheeling and not pedalling. If two cyclists have a similar air resistance, but different weights, then the heavier cyclist will reach a quicker speed. There's the argument that a heavier rider will be bigger and thus have more air resistance, but that effect is smaller than the weight difference. (NB. this can be trivially tested).
