Can someone who sends a text to a car driver who then has an accident while distracted in turn be held jointly liable as a defendant?
Possibly, at least in Pennsylvania and New Jersey, and if the text sender has reason to know that the recipient is driving.
Newsworks has a major story, May 2, here.
it would be logical to extend the question to sending targeted tweets or twitter messengers, if the sender believes that the recipient will get smartphone notification. Possibly it could apply to other apps, including Snapchat, Instagram, or even Facebook (which has its own message notification app).