I do share a Facebook account with my wife, but our use is rather limited. Neither of us post very much, and when we do it is basically to share photos of significant events with our extended family members. We probably average one post per month.
It started as her account, and I would occasionally log on to see something posted by a family member or friend. I did not want to go through the hassle of creating another account and adding all our family and friends, so we just added my name to her account. We do put our name at the end of our posts so friends will know who posted.
I have noticed that some people post every detail of their life, that just seems obsessive to me.