Other than trying to monitor their private life, why would an employer encourage their employees to sign up to Facebook?

Mrs Squeeky works in the health care industry and her employer encourages all employees to get a Facebook page, another friend said his employer is the same, Mrs Squeeky and I greatly dislike Facebook and she wont sign up neither will I, but again other than trying to monitor your private life to a degree why would an employer want their employees on Facebook in the first place?