My wife has similar tendencies and is only on Facebook, but not as large as you describe for your pwBPD.
Outside of the house, she projects the image of a good and healthy family dynamic; is the 'perfect church lady' and the 'ultimate volunteer' similar to that of a social narcissist; however, doesn't rise to the level of NPD. however, behind closed doors it is just the opposite. Classic Dr. Jekyll/Mr Hyde type behavior. However, it also shows that she has control over her behaviors, even if it is at a subconscious level.
...things don't feel that way inside the relationship. What has your experience been with your relationship and social media?