It's actually a common defense mechanism for an insecure person. Offer no independent opinion. Wait until others post an opinion and then pick it apart from the sidelines. Doesn't take much talent to play the role of Monday morning quarterback.

Don't take it personally. Most of the time, people who engage in this practice have a pretty miserable life. Once I get past the my initial reaction of frustration, I usually end up pitying their pathetic life situation.