The use of smartphones may result in drooping jawlines and double chins as facial skin and muscles lose elasticity and begin to sag.

There is still no adequate proof that this condition does exist. Perhaps awareness that it could occur could help regular users take adequate precautions to prevent the development of this condition. Cosmetic treatments like botox, face lifts and chin implants are available. In fact, the number of chin implants has been on the rise in the recent years. And they sure cost much more than your smartphone!
However, it is not necessary to chuck out your smartphone just yet! Firstly, further studies are required to prove that smartphones are actually associated with the condition. Secondly, technology is here to stay and better our lives. A restriction of the total amount of time spent in front of our phones or laptops will enable us to enjoy the technology without suffering from any possible complications. Maintaining a correct posture will also be beneficial. Thirdly, regular facials and toning exercises of the face could help to prevent the condition from developing.
What is probably necessary at this stage is to be aware that jaw drooping can occur with long-term use of laptops and smartphones. A judicious use of these technologies with adequate precautionary measures will go a long way in preventing this condition.
Reference:
1. http://newsfeed.time.com/2012/05/30/dubious-medical-syndrome-of-the-day-smartphone-face/