Send her a certified letter that only she can sign for and you get a receipt that says she picked it up (the P.O will know which one you mean when you describe it).
In that letter, say "Please let me confirm, you are Mrs. xxx, wife of Mr. xxx. I sent you a note via FB. If you received my note via FB and it was truly you that replied to me, accept my apologies for bothering you, but I am suspicious it was intercepted by your H. If you did not receive my note on FB, there is some information I feel I need to share with you".
And go on from there.
That's what I had to do with OMW in my sitch. I couldn't find a phone number for her (she lives 1000 miles away) and I had to hire a PI to get her address. Sent a note certified and was able to go to the P.O. where they confirmed she'd picked it up even before I got the receipt back. She called me that night.
H4U.
Hope4us
Me - 49, W 49 S22 & S18 Dday 9/4/07 W claims NC 4/7/08 8/29/09 - Divorce Busted. Lots to work through, but we're going to make it.