It is possible that you have committed defamation, which could provide grounds to sue you, in sending messages to friends on a social networking site. Defamation is the public making--so to any third parties or other people, like friends--of untrue factual statements which damage a person's reputation. Being accused of cheating would damage reputation. A statement that someone had sex with or cheated with another is a factual statement. So unless what you said is provably true, you may have defamed her. Even if it is true, she could try to sue you, and force you to prove the cheating to defend yourself.