You might want to consider starting out with 10 messages per week (or 3 days, etc.). You're ultimately creating artificial scarcity. If you reduce the supply of messages further and thus increase the price, it's likely you'll see higher quality messages -- which in turn should drive traffic. You can "prime the pump" by starting with a longer message lifetime.

The disadvantage is that you may train users to visit weekly instead of daily, which is obviously counterproductive to driving traffic.