It has been legally defined. See all the people who have lost their jobs over social media posts. Social Media are part of the public sphere by definition. It's a huge public forum. It's like if you stood out on the street and told racist jokes with a bullhorn. Do you think your company would like that?
Yup. Again. HUNDREDS of people have been terminated due to postings made on social media that were brought to the attention of their employers, and it's all legal because they purposefully linked their social media accounts to their employers, causing damage to the employers' reputations and standing in the community at large.
Gilbert Gottfried's termination from AFLAC after posting jokes on social media about the tsunami in Japan is one of the most visible cases of this. He made jokes in poor taste, AFLAC does business in the country that he joked about, and they canned him— even though he's a comedian by trade. Maybe they were being "too sensitive", but they had the right to shitcan him and did it.
Social media is a very public thing. It's not like keeping a diary. Most people who have been fired due to social media posts don't even lock down or make their accounts private. They just post whatever they want, because they think they can without consequence (a la "free speech"/First Amendment). Once you post something for the whole world to see, even if you don't intend to make the whole world your audience, you run the risk of running afoul of at least some of them— and, depending on the severity of the comments, that can lead them to take action.
By now, people really need to start learning how to be more responsible within the sphere of social media. You never know who's going to read what you post, or how they're going to react to it. Think about how others might respond to what you're about to share; if there's even a small chance that you're gonna piss someone off, delete it and keep that thought to yourself.
It's not the best solution, and it might seem unfair, but the fact of the matter is that this is how the world works right now, and arguing that stuff like this should or shouldn't happen won't do much to help you out when you get fired for something you posted on Twitter a few days back. You're still getting fired, and in most states, good luck finding a lawyer who's gonna take on an ill-fated wrongful termination suit (unless you have the cash to pay him/her up front).