Despite all of the media attention, people do not realize that what is posted on the web can be seen by everyone! What to do if a parent bashes you or a colleague? Ignore them! I know it's hard, but you don't want to get into a "verbal" argument with a parent on the web. It makes you, the teacher, look unprofessional and gives the parent more "ammunition" to bash you with.
Download and print off the parent's comments and put them in a file. Share your concerns and the comments with your principal. When meeting with the parent, for parent-teacher conferences for example, have the principal there to provide unspoken support for you. With the principal at the meeting, you might choose to address the parent's personal attacks on you or continue to ignore them. Do what you and your principal feel will be the most beneficial.
Parents forget teachers are human, but know that 99.99% of the parents appreciate what you do even if they don't come out and say it!