Is it worth it to file a suit for slander?

I own a website design business. A past client posted his disgruntlement with my company on a popular website. It'd be one thing if he had vented his anger, or posted facts, but he's made inaccurate and down right untrue accusations about my company. He's painted me as a "skilled con artist", and has made contact with many of my contacts to let them know that I'm not to be trusted and my business is a "front". To date, this has caused me over tens of thousands in lost projects because potential clients have found his post, and then let me know that they were going to pass because of it. I know slander is one of those types of cases that can drag out and is hard to prove, but losing out on big projects like this is pretty troublesome. What's the best thing to do in this situation?