They are called drop tanks. When a drop tank is jettisoned, very likely it is not retrieved and reused. This is also the case with (sadly, discontinued) Space Shuttle external tank. Those tanks were not reused either. I am not talking about the rocket boosters.
What happens to jettisoned fuel tanks?
It depends on where they land. The tanks jettisoned in Vietnam have been used to make boats (more pictures here).
It is worth noting that these external fuel tanks have disadvantage too. From the Wikipedia reference above:
A rule of thumb is that only about half the capacity of a streamlined drop tank actually goes towards increasing the aircraft's overall range, the rest going to overcome the added drag and weight of the tank itself.