Should I list my work as a frequent journal reviewer as a volunteer work in LinkedIn? I am a frequent reviewer for a particular (computer science) journal, and I wonder if the review work I do counts as a "volunteer" work?

I do already list this work in my CV under (PROFESSIONAL ACTIVITIES) section. Should I also add it to my LinkedIn profile under (volunteer experience) section?
I think you should take advantage of being a peer reviewer. You can add it to your LinkedIn account, and create a profile on Publons and ReviewerCredits to be registered as a peer reviewer.