I have never had definitive information that a CEO of a company I followed ever posted on a message board
I have seen at least two CEO's who posted on boards - perhaps 5 years ago the CEO of WiLan posted routinely. And more recently a small biotech I peripherally tracked had a CEO that posted ocassionally (although since it was a peripheral track I can't remember which it was).
And I have seen somewhat lesser lights, even at very big companies, that posted - e.g. a guy who is now one of the 4 or 5 biggest VPs at Qualcomm posted for a brief time perhaps 6 or 7 years ago (albeit before he was a quite such a big shot).