Can I press charges on someone for slandering me on Facebook?

Long story short my fiances ex-wife is slandering us on Facebook. Making ridiculous comments such as child-bride, and yes there is and age difference which she does express very clearly and fully starts are names. She says I am his mistress when she divorced him and had a new boyfriends MONTHS before we got together. Anyways I want to know if I can press charges or sue her?