Dan Schwartz, an employer’s lawyer, wrote a piece about social media. He presents talks on social media and is often asked "is it legal" to "friend" a subordinate? Dan replies, and I agree, that it may be legal, but it may not be prudent. A teacher who "friends" students and employers who friend employees can present awkward situations.
Requests by the employer to "friend" a subordinate could be misinterpreted and could cause the employee to feel pressure. As Dan says, it is better to set these parameters now than after a lawsuit. See Dan’s post for more information.