Yes, no mischief if you accepting the request from your boss through FB. This show he/she indicates feel to know you better separately from your job. Plus, you have built the mutual trust and hopefully the positive outcome from the cyber relationship.
Be reality that he/she stills your boss or superior in organization and have the power to praise and bring you down. Once you accepting the request and please do not post any criticism or bad impression in your organization especially office politic, colleagues and many more. Most of the time people trap by their own attitude. So, what`s the point of freedom to voice your feeling if your boss kept on watching on your post. Think twice before you accept the request.
**Penemuruai`s comment: You and I need scope of privacy.