Is it acceptable to write that you are getting a "Ph.D. in [area]" (rather than name of department) on social media? I am starting a PhD in computer science, and have noticed that a number of classmates will write on LinkedIn or their academic websites that they are getting a "PhD in Machine Learning" or "PhD in Artificial Intelligence" and so on. Is this common/acceptable to write "PhD in [Area of Interest/Expertise]" instead of your home department?
I believe it makes sense.
The PhD education is a very narrow education which will give you very specific knowledge regarding a specific aspect of a research area.
Therefore, it makes completely sense to be a bit more specific rather than general. PhD in computer science sounds very broad and not much descriptive.