I'd say that one, she refuses to take responsibility (else she would have done it on her side); two, she may be "making" you punish/abandon her (because that's what part of her thinks in her mind); and three, she may have done this in a way to force you to connect with her, leaving aside the fact that she's the one who withdrew.
[people with BPD]... .are prone to sudden and dramatic shifts in their view of others, who may alternatively be seen as beneficent supports or as cruelly punitive. Such shifts often reflect disillusionment with a caregiver whose nurturing qualities had been idealized or whose rejection or abandonment is expected.
Asserting boundaries consistently (changing the password, and checking into whatever else you may have given her access to are good boundaries) can help deflect drama. The whole Facebook thing may seem petty and superficial. Maybe since your boundaries are better in other areas, she felt she could violate them in an attempt to assert control again, as she has in so many other ways?
