There was a great post a few weeks back here about why whatsapp, viber et al had such a dramatic growth. Basically, the hypothesis was these apps have access to the entirety of the user's contact data on their phones, which allows them to reconnect with our friends automatically (or fallback to sms/email). Ironically, I think this is something where Google+'s circles could come in very useful for us (meaning, normal users): if Google stopped pushing Plus as an end goal unto itself and instead started integrating it at a lower level in android and provide APIs accessible to other websites, it could have massive privacy and usability advantages for us. Cyanogenmod is doing something similar, but I think they're implementing it as a everything-or-nothing switch for apps. Providing more granularity (and seriously, Google got the circles UI spot on, I'm very impressed) is something that's sorely needed if we want to move away from facebook's monopoly on our social graph.