The first hint of the usage of the word promposal, according to Google, comes from the Canadian newspaper Ottawa Citizen, who apparently held a promposal contest. Unfortunately, the link provided leads to a "page not found" screen. The term makes a minor appearance in 2012 but seems to pick up traction in 2013. You can blame Huffington Post for reviving it and bringing it to the U.S. national stage. Regional interest has been mostly limited to Ottawa, Toronto, and New York, but given the widespread attention of the trend this year, it seems to be catching on elsewhere.
Despite gains in youth's attitudes about gender egaitarianism in relationships, promposals seem to evidence Ellen Lamont's findings (published in Gender and Society) that young women attempt to reconcile egalitarian ideology with conventional gendered courtship norms. While the promposal trend is still developing, how it reinforces or transforms gendered courtship norms remains to be seen.