His fiancée added me on Facebook? Why?

[Ammy]

Crazy that after your husband cheated on you with her (though sounds like she did you a favor by taking him) that she wants to befriend you on Facebook. Only reason I can think of is because of your son, her stepson. If she was sad she didn't' get invited to his party she may want a chance to be invited to future ones.

If you want to be paranoid, maybe it's your ex using her facebook so he can spy on your life. Your description makes me think he was controlling, so seems weird he'd even let his wife have a facebook.

If you message her on facebook and ask her why she wants to be friends on there, I'd be interested in hearing her response.