I don't know about Linkedin, but Facebook and Google+ allow you to set up a "page" or "group" such that you are the owner but other persons (accounts) can be defined as having privileges you specify. That way your intern/employee could maintain the page and when they leave you can simply turn off their access. They never need to know or use your password and you can turn off their access like a light switch. Even better, it allows you to have a public or business presence that is separate from your personal presence.
The trick/difficulty with Facebook is keeping track of their almost constantly evolving/changing privacy rules and settings. But if you have any regard whatsoever for your own privacy and security you should be staying on top of that anyway.
