"A more confident political left [...] has been MIA in recent years."
AWOL, more like. Unless the A in MIA is used in a very broad sense.
"Whether “Digital Millenials” can remember how to talk to the rest of the United States remains to be seen."
I seriously doubt it. Ironically, "social networks" seem to lower social skills. And interacting in a meaningful way* with people outside your regular peer group(s)** is a vital social skill.
* This would include old-school stuff like reasoning, objective facts, politeness...
** Which quite often is really just a bunch of people trapped in the same echo chamber, not people that actually know each other. (Not in the biblical sense, and not counting the odd exception from that.)