How would you handle this Facebook friend/unfriend issue?

I’m friends with a couple who live about 15 minutes from me. They have two children who are my daughter’s age and we’ve all known each other for about 10 years. We occasionally visit and are friends on Facebook. We invite each other to our various parties and also have mutual friends in the local area so we see each other at those people’s houses, too.

The mom is constantly posting on Facebook. In February, I realized I hadn’t seen some posts from her in a while so I looked at her page and notice we were no longer friends.

I had no idea what happened so I called her and left her a vm. She friended me and pm’d me and said she had no idea what happened either.

Now, end of April, again I noticed I’d not seen any of her posts in about a week or two. I looked at her page and noticed we are no longer friends.

Do I let it go totally? Ask her when I see her at a mutual friend’s gathering? Call her or send her a snail-mail note?

It’s inexplicable because we’ve had no communication other than FB (liking and commenting on each other’s posts) so there’s been no fight or anything negative that has happened.

I do know that she is bi-polar and prone to moodiness and drinking. I’m not sure if that may have something to do with this, but again, we’ve not had any negative interactions (ever).