If this isn't a slam dunk real world example of why you should be using content marketing, I don't know that you're smoking.
I just got the results from one of my Facebook boosted post campaigns. For $20, I reached 3388 people with content they are interested in. You just can't reach that number of prospects, nor can you guarantee they have any interest in what you're selling, with old fashioned snail mail.
My fellow Realtors, this is the now and this is the future. You have all kinds of free tools at your disposal to get the word out about your business and increase your income.
Get to work.