Can I be sued for stating my opinion in a Facebook private group about a controversial local public figure?

That is a very interesting question. Generally, if you can prove damages you can sue. How are the postings related to public office? Assuming the plaintiff argues there is no relation and the court buys it this is 2018 and a court might buy that argument. Maybe the damages they claim have nothing at all to do with their public office. Under Rule 11 of the Federal Rules of Civil Procedure (assuming you were in federal court - similar but rules apply in state supreme court) a plaintiff must, to the best of their knowledge, information, and belief, formed after an inquiry reasonable under the circumstances swear that: (1) it is not being presented for any improper purpose, such as to harass, cause unnecessary delay, or needlessly increase the cost of litigation; (2) the claims, defenses, and other legal contentions are warranted by existing law or by a nonfrivolous argument for extending, modifying, or reversing existing law or for establishing new law; (3) the factual contentions have evidentiary support or, if specifically so identified, will likely have evidentiary support or, if specifically so identified, will likely have evidentiary support after a reasonable opportunity for further investigation or discovery; and (4) the denials of factual contentions are warranted on the evidence or, if specifically so identified, are reasonably based on belief or lack of information. The rule goes on to explain that if after notice and a reasonable opportunity to respond, the court determines that Rule 11(b), as cited above, has been violated the court may impose sanctions against the attorney, law firm, or party who violated the above. However, such sanctions are fairly rare and not often imposed. Most attorneys do not bring actions which violate the above rule but it does happen. As for the "public figure" argument the court will evaluate your relation, if any, to the plaintiff, whether you may have breached any duty you may have owed (for example, were your employment somehow associated with the plaintiff that could present an entirely different potential problem) to anyone. You should retain competent defense counsel immediately. Anything is possible in 2018. I suspect we may see considerable precedent overturned in the coming years and free speech may be curtailed a bit. You might face months, years, even a decade or more of litigation which is likely to be costly. I know many who have deleted their social media accounts; especially Facebook. This might be a good time to join them. Lawyer up and good luck!