D., it really doesn't matter what the ex's motivations may be. It is also totally irrelevant to your life whether or not she is in contact with your in-laws. That is her personal issue good, bad, or otherwise.
If you are secure in your relationship with your husband and your son, just relax and let her life take its course independant of you and your family. Whatever her needs and desires are, they don't effect you unless you allow them to. How does it really matter if she is in contact with your in-laws? Your FB page and your husband's are both private and you, and your husband I assume, have not added her as a friend, so just ignore it. And, if your in-laws decide they want to accept her what difference does it make? You don't tell them what to do otherwise so you don't want to make an issue out of the ex either.
You wonder if she is testing you - in what way or for what purpose, I'm not sure. But, if you respond to her IN ANY WAY you have failed the test!
You have, by necessity, a relationship with her for your son's sake. Other than that, there is no reason to even acknowledge her. Treat her as you would any other interloper on FB. Ignore her request for friend status. You know, just because the phone rings it doesn't mean you have to answer it. Just because someone knocks at the door, it doesn't mean you have to open it. And just because someone asks to be invited into your circle of friends, it doesn't mean you have to welcome them.
You are in charge of your life and, whenever you allow something like this to disturb your life, you are relinquishing a piece of that control to someone else. Is she worth it? Do you really want to give this woman control of any part of your life? Do you really want to teach your son that it is okay to let someone else take control?
Probably not.
So you really know what to do, don't you? Nothing!