They are variations on the social media website world. First there was Myspace, then came facebook, then twitter, and then the online world exploded into a macrocosm of information and digital social interactions; wordpress, tumbler, flickr, etc. Pinterest, Pinspire, and weHEARTit are simply websites that use photographic content as their staple media to convey subjects around which people can comment and converse.
EDIT: I do not have an account on any of the 3 websites mentioned in the title of this thread, I am just familiar with them.