I don't know anyone who uses them - or at least, nobody has ever suggested I use them. And I haven't been interested enough to try myself to consider converting other users. I've got enough communication routes already without adding a new one unless it can either replace one of the current ones (I won't get all my friends who use whatsapp to switch so it won't do that) or I need it to talk to a subset of people I know (not the case currently with hangouts - this was why I started using whatsapp in the first place, though)