I think a fairer solution will be some form of proof of personhood that isn't PoW-based. Your idea isn't bad but it gives more power to those who can afford a lot of devices. You know those Chinese mobile phone click farms they use to game app stores? It will be like that, PoW can prevent spam only to a certain degree and with all the social media and networks we have today there is a lot of money in influencing the users. So spending a few million dollars on devices can be very profitable if it lets you boost certain messages.