Are we talking about whether it's morally defensible to shame someone for doing something irresponsible (like texting and driving) or whether it's actually likely to change their behavior (ie. whether it's good for society).

The latter does not seem to clear cut; there are likely better, more positive ways to change behavior. Shame is more likely to make the person get better at hiding their irresponsible behavior, which is actually counterproductive for society.

I'm still not convinced that shame is ever a useful tool.

I can see why would you care about texting and driving (and why there are laws against it). Why would you care about selfies?