MANA was initially created to be deflationary. Initially, the supply of 2,805,886,393 was burned in the Terraform event, the second auction, and the DAO creation. But there are other ways in which MANA gets burned.

Any activity on the marketplace charges a 2.5% fee on the sale (payed by the seller). This means that the amount of MANA burned is directly tied to the activity of the platform.

The less mana there is, the more valuable each token becomes. Any marginal demand for MANA would cause an increase in the MANA price, leading to an appreciation of the token.