I'm not sure if it does.

These complaints/compliments towards a specific crewmember do have an effect on their status within the company. So it is only logical that KLM would want to verify the authenticity of such messages by asking for (eg) a ticket number.

Without that information (like on Facebook) how can they guarantee its not simply family member(s) of the crew posting good comments, faking the system?

Personally I suspect that posting these things on facebook will not have the same effect as a formal one through the website. For which I am pretty sure by the way that no bank details are required (Name and ticket number is all they need pretty much).