I think it's not about social acceptance. It is unacceptable because if you call them bullshit without giving good reasons why you think so it is not advancing the discussion in any good way. You are not contributing anything except your personal opinion.
If you do have good reasons to explain why you think they are wrong you don't need to call them names because you can just provide the correct reasoning instead.
I think something to reiterate is that it never goes well when you tell the other person that you disagree with them. A person is not just one idea, they're made from thousands... You're merely disagreeing with one idea, and that should be made clear when you're speaking with them. Regarding the parent comment's terminology of "calling bs", that's a bit of a harsh way to put it, or approach it, if you're trying to reconcile a difference of opinion over a single idea, and not fundamentally pointing at another person and saying, "You are your one bad idea and I don't like either."