I would advise against it. I don't put it on my resume because many people have strong negative feelings toward the organization (usually ignorantly) and it is not worth my time to explain to everyone who sees my resume that "no, I am not an extremist right-wing republican, no, I don't hate women and people of color," etc. Unfortunately, disinformation about fed soc is pervasive in law schools and the legal community in general.
An alternative reason not to put it on your resume, is that it does not really add anything to your credentials. It merely makes a political statement. Generally, your resume should reflect what you have achieved and joining an organization is not really an achievement.
Finally, ACS probably would not receive the same response. One, because it is of a lower profile than fed soc and two, to state the obvious, liberal things are far more acceptable in law schools than supposedly conservative things (although fed soc is not necessarily conservative--once again, though, I hate explaining this to people).
Were you at the student convention by any chance?