A common question is whether and when to engage your critics. It is the wrong question.
Asking that question implies the conversations by critics depend somehow on your involvement. You may have stimulated their criticism with something you said or did, but their conversation is proceeding on its own propulsion, whether you are engaged or not.
The right question isn't "whether and when;" it is "how and where."
Not all criticism merits frontal confrontation. Someone who voices his or her displeasure on your Facebook page might be dealt with best offline, especially if you can satisfy the complaint quickly. That person is likely to follow up his or her own critical post with a laudatory one about you.
Sometimes critics need to be confronted. They may be spouting inaccurate or distorted information about you. That requires your direct engagement at the point of criticism or anywhere else that misinformation may have been planted.
People always have talked behind somebody's back. The only thing that has changed with the advent of the Internet and social media is that those conversations occur right in front of your face.