What are the characteristics of effective K-12 teachers' personal learning communities (PLCs), and can they be digital?
I think that for PLCs to be effective, they need 3 things: focus, responsibility and authority. The careful balance of these three results in teachers who are engaged, who have a sense of purpose, and who can influence the school's culture. Most importantly, the PLCs need to cultivate individual's and/or the school's social capital.
PLCs will be more effective if they have a particular focus, whether its departmental (science or history teams, pedagogical (instructional or assessment strategies), operational (grant$ review or school culture) or interest-based (shared readings or other).
Effective PLCs need to have specific responsibilities such as completing a project, reporting back to the larger faculty and/or making policy recommendations. Responsibility implies accountability, and the interaction between PLCs fosters accountability. Schools need a deep leadership bench, and teachers can contribute to a school's strengths (and their own strengths) via PLCs.
A PLC's sense of authority is a tricky one. Some teachers or administrators dispense authority to groups or individuals without proper and gradual experience with cost-benefit analysis, diplomacy, or communications etiquette. If this is true, one solution is to have clear organizational goals and to understand how a specific PLC's aims fit into or advance the larger goals. PLC members also need to develop 6, 12 and 18 months goals (more or less depending on the project) so that they can gradually raise the general social capital of the school overall. PLC's authority need to be honored and administrators, in particular, need to make sure they have provided enough context so that the PLCs understand the implications of their choices and recommendations.
On a side note, PLCs are more likely to be effective when administrators are participants.