The problem is that the SEC cracks down hard on "insider information." The point being, if in a private meeting with you, the CEO told you they did something über cool and you used that to buy stocks, then you had info someone outside the company didn't, and that is a violation of SEC rules.
The question here is whether or not posting on a facebook page is a "private" matter. If they posted this in a PR release (or, recently, on their website) for all to see, it would be one thing. But posting it on a facebook page where by default only the subscribers to that page would see it, the SEC suspects it is an unfair disclosure.
To expand: Imagine if the CEO of the company didn't use Facebook publically, but only had friends, family etc. on there, maybe 200 people. And then posted there first. That might count as an insider trading / private.
Like Apple announcing how many iphones they have sold? Or how many apps have been downloaded to date?
What do I misunderstand?