When somebody puts up a facebook status, email or message of any kind and asks you either to send it on to others or repost it. Once you have done this you have successfully achieved bandwagonism! It is usually to spread awareness of some kind but actually does nothing whatsoever in order to help the cause. Only done by people with nothing more interesting to say and usually have no knowledge of the cause itself.
The NSPCC change your facebook profile picture to a cartoon for abused children campaign. Lots of people were guilty of bandwagonism.