First, I don’t know if Laurel happened upon a group of Luddites, but I’m going to assume for now that the situations she describes isn’t unique.
If that’s the case, I would imagine the reason why grad students aren’t big on Twitter is that their word’s current judges – their professors – are their key audience and they don’t need to use Twitter to reach them. Email, phone, or physically visiting their professors in class or during office hours is probably sufficient.
But when they’re out of school and trying to reach a much larger audience, they may find more use for communication technologies that scale. All of a sudden, it becomes physically impossible to email, phone, or physically visit everyone you’re trying to communicate with.
Hopefully, the students realize that their lack of need is situational and will likely change over time.