I agree with jrobconsult, with postcards, these are immediately noticed by the recipients. No need to open envelopes before you get to the message. A good call-to-action message, a promo, an exclusive special offer, will definitely make the reader attracted.
Indeed there are many companies that are cutting back on postcards advertising, but there are also some who are taking this as an opportunity to tackle the market by continuously using the strategy. People will appreciate getting a well-designed postcard on a nice paper stock, especially since they know it probably cost more than other advertising media.