My experience with postcards indicates they are the best used as a second, third or even fourth hit. The one big advantage of the postcard is you don't have to worry whether the envelope will be opened, which is always the case with a letter.
Nevertheless, a letter gives you the opportunity to tell a longer story. In print advertising, the more you tell, the more you sell. You may wonder sometimes how marketers can have the audacity to send you an 8 page letter. There's a reason for that. It gives more space for marketers to tell a story. The more space available, the better the chance to convince the reader that you're selling is worth a response.
There's an old saying I subscribe to, "there's no such thing as a letter that is too long, only a letter that is too boring".
The truth is, if the person reading your letter is interested in your product or service, they'll read whatever length piece you send them. If they're not interested, they're not going to read it regardless of what's in it. If it's boring you're not going to keep their attention long enough to give them a chance to read the rest. Don't worry about how long the letter is. In my opinion, the longer the better, as long as its not boring.
In the marketing I've done, I've found that longer usually sells more. A two-page letter is usually out-sold by a four page letter. A four-page letter is usually out-sold by an eight-page letter. An eight-page letter is usually out-sold by a sixteen page letter. Believe it or not, the best producing letters I've ever seen were thirty-two pages long!
Both postcards and letters can be used to target prospects from FSBO signs, FSBO magazines, cross-referenced newspaper ads, expired listings, etc.
Expired Listings -
How do you get your hands on expired listings? Any agent can pull up a list of expired listings from the MLS computer. A well-crafted letter to these sellers should produce calls from people looking for solutions. Mail the letters at least twice monthly and follow up with a second mailing to the same list two weeks later. If you still haven't received a call, send a post card two weeks after the second mailing.
You should mail the first letter immediately after the listings expires, or better yet, 30 days before the listing expires.
A little word of caution: You'll need a good relationship with an agent to get access to MLS listings. This service is paid for by licenses and meant for their use only.