Your question concerns "forum shopping," which belongs to the field of study.called "conflict of laws" or "private international law." It is probably the most complex area of study within the law and has a scholarship extending back over 800 years. One could discuss the International Shoe test or Pennoyer v Neff, cases which every law student learns, the due process clause, state long-arm statutes, forum selection clauses and whether or not they are binding, and many other things. We could talk about the first and second Restatements of the law on Conflict of Laws, Joseph Beale, Joseph Story, multilateral rules, the conflicts revolution, and "interest analysis." But even if we held classes on the subject three times a week for four months, with voluminous readings between classes of relevant cases, books, statutes, and law review articles, I cannot guarantee that at the end of that process you would emerge with a solid understanding of this arcane and mysterious subject.
The proper place to sue depends on many issues. It could be the location of the plaintiff, or the location of the defendant. It could be the location of the incident giving rise to the claim. It could be the location where a contract was entered into. To determine proper venue requires a review of many factual elements that you have not provided.
The plaintiff can choose to sue you in your home jurisdiction or where the harm occurred. So, lets just say for the fun of it that they are in Florida. They sue you in Florida. The Florida court will look at Florida's "long arm statute," which pretty much says that if there is the slightest reason to file in Florida, it fits the long-arm statute. Other states have less expansive long-arm statutes. For example, New York excludes long-arm jurisdiction for defamation claims. (I raised this because you noted that one of the areas of law here is "slander").
After that, the inquiry shifts to whether asserting jurisdiction comports with due process. Essentially, "is it fair" to sue you in Florida? If the court determines that it is not, the case will be dismissed for lack of personal jurisdiction. Much of that inquiry is about whether the harm the plaintiff felt was reasonably forseeable in the forum state.
If you are the potential plaintiff, then you should consider whether the defendant knew you were in CA at the time, and whether the defendant knew that the brunt of the harm would be felt in CA. The seminal case on this issue is Calder v. Jones. (involving a CA plaintiff and a FL defendant in a defamation case). However, you might also want to consider the strategic value of filing where the other party is. CA is a bad place to be a defamation plaintiff. So, if you want to escape from spending time and money on a motion to dismiss for lack of personal jurisdiction, and you want to avoid getting hit with an Anti-SLAPP motion, you might want to look at the defendant's home state.
The other lawyers have covered this very well. The one variation involving the Internet is this beast known as Terms and Conditions. Nobody reads them, but they can be quite onerous (a point South Park went to town on with respect to Apple's Terms) and often include jurisdiction and even venue provisions that might be controlling.