The longer a couple has been together the more likely its partners are to share their online accounts, that's according to a Pew Research study on couples sharing online accounts and passwords.
While it makes sense that most couples don't share a Facebook account, many do share an email address.
The thing most commonly shared by online couples is passwords. More than half of the couples in the survey stated to know each others passwords regardless of how long they have been together.
You will find more statistics at Statista