Free speech and the workplace intersect in complicated ways. Outside the workplace, you can say pretty much anything (with few exceptions) about anyone. Insider the workplace, by contrast, you're limited -- both practically and doctrinally -- in what you're categorically permitted to say without potentially adverse consequences.
But those spaces intersect. Many of the people you interact with on a daily basis are those you meet at work. What can you say about them in your private sphere; e.g., on Instagram?
It's a toughie. As this Ninth Circuit opinion amply demonstrates.
Here, the Ninth Circuit holds that it's potentially a Title VII violation (hostile work environment) for one co-worker to make various derogatory posts on his Instagram page about another co-worker. Read more from Judge Wardlaw if you wish.
I'm just now returning from a family vacation, so don't have much to say about the opinion other than it, at a minimum, demonstrates a practical reality:
Be careful what you publicly say about your co-workers. Your freedom in that sphere is not nearly as expansive as what you're able to say about, say, presidential candidates.