A person's right to free speech is more limited than most folks realize. We cannot utter threats, especially if we give others reason to believe their person, family or reputation are at risk. We cannot necessarily speak freely on private property, if the property is not our own and the owner disagrees with us -- which is where we stand with the internet. People who say something that is threatening or defamatory on social media should receive the same punishments that would be received if they issued those comments on a street corner.
If people engage in threatening or otherwise harmful behavior, they should be arrested and the threat should be taken seriously just as if they had said these words in real life. Just because they are said online does not make them any less real or any less harmful if they were said offline.
No, people should not be arrested for what they say on social media. People should be free to express themselves on social media unless they are making direct threats. If these are the people's honest opinions, they already think that way whether they tell anybody or not. Thus, arresting people for what they say on social media will only keep people from expressing themselves.
Freedom of speech extends to social media. People should be allowed to express themselves and say what they believe without fear of reprisal. If people are unhappy with other people's comments, they can simply block them. The only exception is if a person uses social media to make a serious, direct threat. In this case, the law should get involved.