From the wiki:
https://ripple.com/wiki/Ripple_Introduction"Everyone who uses ripple has a balance of credits. When transaction fees are paid, the fee is destroyed. This is essentially the same as redistributing the fee proportionately to everyone with a ripple account."
This will incrementally increase the value of all ripples.
The default fee is .00001 XRP and there are 100 billion XRP in existence. Since the ripple genesis ledger less than 1 XRP has been destroyed -- they won't run out.
The fee is meant to discourage spamming and prevent DoS attacks from killing the network. As network congestion increases, the fees will go up dramatically; otherwise, they will stay almost non-existent. When an attacker attempts to spam the network, fees will skyrocket, and they will soon go bankrupt. When the affected node(s) recover, fees will return to normal.
